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Detective Mole

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Among the various cops, forensics people, etc., working on a crime scene in fiction, there's a good chance that one person on the team will know a lot more about what happened than everyone else present... because they committed it.

In real criminal investigations, it's not uncommon for the offender to somehow attempt to insert himself/herself into the investigation to keep tabs on it, or for ego's sake. In this trope, it happens that the offender is actually an official part of the investigation, and is quite probably cheerfully leading Detective Patsy astray as much as possible.

This applies to any crime. The defining characteristic is that one of the investigators is in fact the perpetrator, whether the audience knows it or not. Compare Bruce Wayne Held Hostage, Hired to Hunt Yourself. When the Detective Mole is very high-ranking, see Mole in Charge.

No connection whatsoever to Hanna-Barbera cartoon star Morocco Mole, who is an assistant detective who happens to be a mole.

This was explicitly banned in Ronald Knox's "Ten Commandments" of the Fair-Play Whodunnit story, although some authors viewed the prohibition as only applying to the main investigator and still wrote stories unmasking a secondary member of the investigating team as the criminal.

Related to The Mole.

Warning: This is a Spoilered Rotten trope, which means that EVERY SINGLE EXAMPLE on this list is a spoiler by default and most of them will be unmarked. This is your last warning, only proceed if you really believe you can handle this list.


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    Anime & Manga 
  • Death Note. It isn't a particular murder, but a massive supernatural killing spree...but the killer is still called in to investigate.
    • Though this is because L, the lead detective is the world's best three detectives, and has called in Light, both because he is useful as a detective, and because he has a high probability of being the killer, presenting the chance for the lead detective to get into his mind. This was his stated purpose in bringing in Light.
    • As a twist, during a good part of the investigation, Light doesn't even know he's the killer.
    • Played straight in Death Note: Another Note. Naomi Misora is very impressed with Ryuuzaki's ability to decipher Beyond Birthday's codes when no one else can—of course it's because "Ryuuzaki" is Beyond.
  • Runessa in StrikerS Sound Stage X of Lyrical Nanoha, who's both part of the investigation group on a killing spree and the true mastermind behind said killing spree.

    Comic Books 
  • Inverted in one Green Manor story, where the detective looking for the elusive Serial Killer John Smith (who has baffled police and profilers alike, because he never seems to have the same MO or psychological profile, other than leaving "I will kill again" at the scene) realizes there is no serial killer: every crime was committed independently and signed John Smith. So as to bring an end to this universal alibi, the detective exposes himself as the killer.
  • In the first Marshal Law mini-series Fear and Loathing, the serial killer the Sleepman turns out to be Danny Mallon, Marshal Law's apparently wheelchair-using Mission Control.
  • In the Spectacular Spider-Man Sin-Eater story, Spider-Man works with detective Stan Carter to try to unmask the serial killer, only to discover it's Stan who's the bad guy.

    Fan Works 

    Film — Live-Action 
  • In Ace Ventura: Pet Detective, a murder and two kidnappings (one dolphin and one Dolphin) are investigated, and also instigated, by police chief Lois Einhorn.
  • The Big Clock: After he murders his mistress Pauline York, publisher Earl Janoth decides to frame the man he saw leaving her apartment before he arrived, whose name he thinks is Jefferson Randolph. To his end, he orders the staff of Crimeways magazine to devote their efforts to locating Randolph, and puts Crimeways editor George Stroud in charge of the hunt: not knowing that 'Jefferson Randolph' is really George Stroud.
  • In Breach, O'Neill's supervisor mentions that Hanssen was the head of a task force charged with ferreting out the mole, who was him. Hanssen also alludes to this in his Motive Rant at the end.
  • In Captain America: The Winter Soldier, when Nick Fury is attacked and believed killed by the Winter Soldier, Fury's direct superior, U.S. Secretary of Defense Alexander Pierce, personally takes over the murder investigation. One problem, though: Pierce is secretly the head of HYDRA, and is the one who put out the hit on Fury to begin with.
  • In Chaos (2005), The secret leader of the bank robbers and the one aiding their plans is none other than detective Quentin Connors, who was assigned to the case.
  • In Cobra, one of the police officers protecting Ingrid is a member of the New Order.
  • Confidence: Special Agent Gunther Butan turns out to be this for Jake.
  • In The Departed, Matt Damon plays a Boston cop and mole for Jack Nicholson's criminal empire who is charged with finding the mole in the Boston police department (himself). The Departed was an adaptation of a Hong Kong film titled Infernal Affairs.
  • Don: The Chase Begins Again: De Silva, who is, in fact, Vardhaan and a rival drug lord to Singhania.
  • In Flash Gordon (serial), Ming tasks his High Priest of Tao with uncovering a traitor who provided Flash information. The Priest is the one who did so.
  • A Gun in His Hand: A man enrolls in the police academy, graduates at the top of his class, and becomes a beat cop—all as part of his real job, as leader of a gang of thieves. He uses his inside knowledge of the police force to commit a series of lucrative heists.
  • Horns movie adaptation features a variant of this. Ig Parrish is accused of the rape and murder of his girlfriend, which had been committed by Lee, who is Ig's best friend and lawyer. Lee actually tries to frame Terry (Ig's elder brother) as the culprit.
  • The Infernal Affairs Trilogy: Ming was tasked by Wong to find out the mole within the police force, which also happens to be himself.
  • Investigation of a Citizen Above Suspicion opens with the Villain Protagonist killing his girlfriend. He then goes to work—at police headquarters, where he's chief of homicide. Moments later he's called out to investigate the killing he just committed.
  • In I See You the disappearance of a boy is thought to be a copycat of crimes committed years before, for which a man was imprisoned. It turns out that the detective investigating the case (the main character) is responsible for all of them.
  • In Next Time I'll Aim for the Heart, the protagonist is a French serial killer named Franck Neuhart, who is also one of the gendarmes investigating this very murder spree. The story is based on the real story of a gendarme named Alain Lamare (cf. Real Life).
  • Knives Out: An instance where it is not the main detective leading the investigation, but The Watson. Blanc enlists the help of Marta to aid in his investigation, all the while she was under the impression that she was the murderer and works to muddle the clues every step of the way. In this instance the trope is also Played With, in that Blanc suspected her all along and chose her as Watson because of it.
  • Played With in The Last of Sheila. In Philip's final deduction of the real truth behind Clinton's murder and Lee's suicide, Philip recruits Tom as his Dr. Watson, not realizing at first that Tom is the actual killer. Once he figures out that Tom was behind it all, he drops the facade entirely.
    Philip: I've had the beginnings of an idea, too, for a scenario. It's about a middle-class writer who is married to a beautiful and wealthy young woman....After a few years, she begins to bore him. Not her money, she herself. And then, while rewriting a picture in Rome, he renews his acquaintance with a cheap, but...Not untalented young actress.
  • In Maniac Cop, Sally Noland, who was Cordell's girlfriend before he was arrested, feeds him information from the inside.
  • In Mr. No Legs, Captain Hathaway is Chuck and Andy's boss, and also secretly D'Angelo's partner in crime.
  • In Nightwatch (1994), the detective is the Serial Killer.
  • In No Way Out (1987), the three main characters involved in the investigation of a Pentagon-related murder that is blamed on a supposed Soviet mole are each guilty of a crime. One is the actual murderer, another is helping him frame someone else for said murder, and the third is an actual Soviet mole.
  • In Ocean's Twelve and Ocean's Thirteen an Interpol agent (in 12) and the main FBI agent investigating the gang (in 13) turn out to be respectively Linus' mother and father, and actually help the con.
  • In One Foot in Hell, Sheriff Mitch Barrett masterminds the robbery of the Blue Springs bank and then leads the Posse to hunt down the bank robbers. His accomplices believe he will steer the posse away from them, but he actually does the opposite: hunting them down and killing them so he doesn't have to split the loot.
  • Original Sin 2004: Walter Downs, the detective who investigated Julia, told Luis that she may be working with someone. He was said someone all along.
  • Several times in Pathology members of the murder group perform the official autopsies on their victims, allowing them to easily disguise the real cause of death.
  • Point Of Origin: John Orr is a fire investigator who unknown to all has been setting a series of fires. Unaware of him being the arsonist (at first), the task force consults him, brings him to the crime scenes and even asks him to create a psychological profile of the arsonist.
  • Saw: Detective Mark Hoffman is an apprentice and the eventual successor of Serial Killer Jigsaw, who is implied to have had the major role of finding crimes for victims, and at one point had warned Jigsaw of incoming cops that could stop him. He likely had dropped from these roles by the time he was revealed in Saw IV, however, as Jigsaw had died by then.
  • With the exception of John Hartigan, every cop in Sin City is working for the bad guys. The only difference is that there is no reason to keep it a secret.
  • In The Sting the FBI agents who assist Lieutenant Snyder in his pursuit of Robert Redford and convince him to help their investigation are revealed to be the final part of the sting.
  • In Summer of '84, a group of teenagers believe the Cape May Slayer to be one of the police officers working on the case. They're right, and the officer uses his position to arrest someone else for his crimes when the kids get too close to the truth.
  • In Vicki, Lt. Ed Cornell knows who the real killer is, but he is so hopelessly in love with the dead girl Vicki, who herself despised him, that he intends to railroad an innocent man to the electric chair.
  • Judge Doom is ostensibly the one looking for the one responsible for murdering Marvin Acme in Who Framed Roger Rabbit. He's the one who committed the murder. What's more, he also kills R.K. Maroon before he spills the beans. And was the one behind the company buying Maroon Studios and the Red Car. And was the one who killed Eddie's brother all those years ago. Moreover, he did all this so that he could destroy Toontown and build a resort area next to the soon-to-be-completed freeway. (And, according to one of the first drafts of the script, he was the one who shot Bambi's mother!) That's one seriously disturbed toon.

  • In The ABC Murders, after the third murder, Poirot puts together a group to solve the murders, consisting of himself, Hastings, and one or two people close to each of the victims. One of these is Franklin Clarke, the murderer.
  • In Angels Flight, the murderer turns out to be one of the detectives on the LAPD task force investigating the murder. He uses his inside access to tamper with evidence in order to frame a different cop.
  • In Another Note, the private detective Ryuzaki, who assists Naomi in her investigation and who the audience naturally assumes must be L, is actually B himself. He steers Naomi to recognize the clues he wants L to see and plans to use their stakeout as a setup for his final "murder" of himself.
  • In Arsène Lupin in Prison, the Inspector Ganimard from the first half of the story, hired by Baron Cahorn to stop Lupin's newest scheme, is an impostor employed by Lupin, as opposed to the real Ganimard who visits Lupin in jail in the second half of the story.
  • In The Black Echo, the FBI agent in charge of the task force investigating the bank robbery turns out to be the mastermind behind the bank robbery, and the murderer to boot.
  • In Georgette Heyer's detective novel A Blunt Instrument, the killer is revealed to be PC Glass, the local beat policeman who "discovered" the victim. The titular weapon was his official truncheon.
  • The Cartel: Gerardo Vera, while he maybe fighting the cartels, he is fighting on behalf of the Sinaloa Cartel.
  • The murderer in The Caves of Steel is in charge of the investigation, and specifically picked his old friend, the main character, to handle it.
  • CHERUB Series: Michael Patel, who is a Dirty Cop, tampered with evidence to cover up a murder that he committed.
  • Commissaire Adamsberg :
    • In This Night's Foul Work, Ariane Lagarde, a highly-respected forensics expert deliberately misleads the whole investigation team by setting up a false but very convenient culprit. Adamsberg and Danglard do think that someone connected to the crimes is in their team... but they spend most of the book suspecting Lieutenant Veyrenc.
    • In The Ghost Riders of Ordebec, the Ordebec murderer is none other than Louis-Nicolas Emeri, the gendarmerie officer initially in charge of the investigation.
  • In Curtain the murderer is Hercule Poirot himself. Well, he's *one* of the murderers, anyway—still not the main one.
  • In Desert Star, one of the civilian volunteers on the Open-Unsolved cold case squad is an ex-cop who also happens to be a Serial Killer. In fact, he knew that Open-Unsolved was going to investigate a murder he committed, so he got himself on the squad to keep tabs on the investigation.
  • In Val Mcdermid's The Distant Echo, four students literally stumble upon the body of a young woman, and are immediately suspected of the crime. The reader is led to wonder if it was one of them who killed her and in that case who. As it turns out, the real killer is the first police officer on the scene.
  • Esther Diamond: Combined with Living with the Villain: in Disappearing Nightly, with said Mole even taking the leftovers from the meals that Max, Esther the rest of the Amateur detectives had while investigating his crimes and used them to feed his prisoners!
  • The Executioner:
    • A couple of times, Bolan will find a high-ranking cop or federal agent has been on the pay of the mob for years to cover their tracks.
    • The spinoff Phoenix Force novels have an entry when the Force helps a joint Miami PD-DEA operation to bust a drug cartel. Before the raid, one detective and a DEA agent get into a shouting match that nearly becomes a fight. Just as the raid is about to begin, each of those men tries to kill their fellow cops, only for the Force to stop them. Both men had been on the cartel's payroll and thought the fake fight would make them look like they were dedicated to the job. In reality, it made them look like unprofessional punks and made the Force suspicious.
  • Fantômas: In The Hanged Man of London (1911), Fantômas himself has infiltrated Scotland Yard and serves as one of its top detectives.
  • Gervase Fen: In Buried for Pleasure, the murderer is the local police superintendent.
  • In Hercule Poirot's Christmas the murderer turns out to be Superintendent Sugden, the local police officer involved in investigating the murder.
  • Killing Time: The attempts on Tim's life are all committed by the chief of police conducting the investigation.
  • In the Lord Darcy novel Too Many Magicians, the true identity of the murderer/Polish spy known as 'Goodman Fitzjean' is Commander Ashley, the Naval Counterintelligence officer assigned to track Fitzjean down.
  • In the Henri Bencolin mystery The Lost Gallows by John Dickson Carr, the Big Bad Jack Ketch turns out to be Sir John Landervone, a retired senior detective from Scotland Yard assisting the investigation. Sir John had appeared in previous Bencolin stories as an ally, so his Face–Heel Turn comes as more of a shock here.
  • Mackerel by Moonlight, whose real claim to fame is that the author was governor of Massachusetts until right before it was published.
  • In The Murder of Roger Ackroyd, the new Watson figure, who replaces Hastings as Poirot's sidekick and the novel's narrator, is the killer.
  • In Gaston Leroux's novel The Mystery of the Yellow Room, the murderer turns out to be Frederic Larsan, the Lestrade-like police detective who competes with the Amateur Sleuth for solving the crime.
The Overlook: One of the FBI agents chasing supposed terrorists is actually the bad guy.
  • Paul Sinclair: Lt. Silver gets assigned to gather witness statements in regards to an explosion and fire for the investigation. Since the incident was his fault, he makes a point of discarding all statements that could lead to the investigator learning that his actions caused the situation that made the explosion possible before turning them in.
  • In The Poet, the Serial Killer that the FBI is hunting turns out to be one of the FBI agents.
  • Michael Slade's RCMP novels are fond of this trope, using it with a Mountie in Headhunter and a forensics analyst in Primal Scream.
  • Roma Sub Rosa used this once.
  • The Savannah Reid Mysteries: In Cooked Goose, after Titus Dunn commits his crimes, he helps investigate them. Until, during one of them, he gets injured in a car crash - then he "goes missing".
  • In The Secret Adversary, Tommy and Tuppence are trying to locate a secret treaty before it falls into the clutches of a criminal kingpin known only as "Mr. Brown", who will use it to harm the British Government. They are joined by Sir James Peel Edgerton KC, who actually is Mr. Brown.
  • In The Secret of Chimneys, M. Lemoine of the French police turns out to be King Victor, the international jewel thief, who has kidnapped the real Lemoine and is impersonating him.
  • In The Seven Dials Mystery, No. 7, the leader of the Seven Dials secret society, is revealed to be...Superintendent Battle. Played with when it turns out that the Seven Dials are actually good guys, an amateur secret service working under Battle's direction.
  • The Shadow: This happened when a police inspector was covering up evidence at the scene of crimes his organization had committed until he was stopped by the Shadow.
  • In The Stormlight Archive, Prince Adolin kills Highprince Sadeas for trying to usurp Adolin's father, knowing that Sadeas left no proof and would be too highly ranked to convict normally. When the body is found, Adolin's father assigns him and his fiance Shallan to investigate. Adolin reluctantly goes along, but they're all quickly distracted by the coming war.
  • In Three Act Tragedy, the culprit is one of the Amateur Sleuths whom Poirot and Mr. Satterwhaite collaborated with.
  • Time Scout: Sid Kaedermann insinuates himself into the rescue mission for his own victims.
  • Traces: All the evidence in Framed! points to the case's investigator, Luke, being the culprit. Subverted in that he's innocent and proving it is the impetus of the novel.
  • In the Vorkosigan Saga novel Memory, the ImpSec head Simon Illyan is poisoned by Lucas Haroche, one of his direct subordinates and effectively his second-in-command. Haroche doesn't lead the investigation personally, but he does decide who is on it, its scope, and who can contact Illyan.
  • Ivan Frantsevich Brilling in The Winter Queen. He wasn't directly involved in the crime itself, but it was done by members of the same organization.

    Live-Action TV 
  • 24: Live Another Day: Ken, a Detective Inspector with the London Metropolitan Police. He is on the CIA's payroll, providing them with classified police information that pertains to their activities and investigations.
  • Happens several times on Babylon 5, due partially to the fact that the eponymous station is where folks go (at least earlier in the show) when they get Reassigned to Antarctica. Many of the security guards work there simply because they had nowhere else to go, and more than a few are easily bribed. The most notable example, however, would be Garibaldi's assistant Jack, in the first season.
  • Backstrom: Arson Investigator Samantha Orland is the arsonist in "Bella."
  • In Bäckström (2020), Bäckström is offered money by the Russians to feed them info, or else be killed. He breaks down and, begging for his life, literally grovels at Boris' feet, before pulling out a gun and making short work of them... and then takes the money for himself anyway.
  • Banshee: In the season two premiere, Lucas gets called in to investigate the robbery of an armored car from the Indian casino. The same one Lucas, Carrie, Sugar, and Job knocked over earlier in the episode.
  • Bones
    • Kenton in "Two Bodies in the Lab" is an agent working on a case with Brennan and he tries to kill her when the truth comes out. Booth's Roaring Rampage of Rescue saves her just in time.
    • The tech expert in "The Brother in the Basement". He helps Angela go through a suspect's computer but is really distracting everyone to try and keep them from realizing he's the killer.
  • Blue Bloods:
    • In "To Tell the Truth," Linda is kidnapped in an attempt to intimidate Danny into not testifying against a drug lord whom he witnessed executing a subordinate. It becomes clear that there's a mole who leaked information about Danny to the drug lord's network. In the end, it's proven through some digging from Frank's secretary Abigail Baker that the mole bought burner phones for the drug lord, identifying the federal agent who sold Danny out.
    • In "Working Girls," Danny and Jackie are assigned to protect a witness scheduled to testify against a Russian mobster who killed an associate's wife, due to Erin's previous star witness (said associate) being killed off by the mobster's henchmen. They hide her in a hotel room, but the room is compromised when Danny realizes that the repairman sent to fix a broken air conditioner is actually a hitman. Danny realizes that there's a leak from the task force. He has Jackie stash the witness at her apartment, while he goes over the files of everyone on the task force to look for the mole. With nothing turning up, he and Gormley leak a false address out to the task force, hoping that the mole will react accordingly. Sure enough, he sends two hitmen to the address, and break into an empty apartment occupied by an ESU team. The hitmen quickly give up the mole who fed them the information.
    • Subverted in "Above and Beyond". Steve Tomlin, an undercover detective, is killed after the drug kingpin he's investigating finds out his true identity. When Gormley and Danny go to empty Tomlin's locker, they find it's been emptied and it's suspected that the dealer may have a mole. Turns out there isn't a mole, and Tomlin's locker was emptied early at his request because it was going to be discovered that he was cheating on his wife with another woman, with whom he'd fathered a kid. And it was his wife who accidentally blew his cover, calling a guy in the dealer's crew mistaking him for Tomlin's mistress.
  • Michael in Burn Notice does this often when he gets hired by the bad guy to find out who is after him (more often than not being Michael himself). One notable one is when he is hired to be a Spy Hunter and pretty much has a blast suitably suggesting it could be him to the mark.
  • In one episode of The Closer, a man who killed two of his girlfriends in the late '90s (but was never convicted because the bodies weren't found) is murdered. The man who lead the first investigation, a Detective Olin who has since retired, is asked to come in and help them with the new murder, because of his insight into the original case and his close relationship with the families of the victims. In the end, it turns out Detective Olin is dying and, desperate to solve his That One Case, killed the man after torturing him until he revealed where he buried the bodies.
  • In the Columbo episode "A Trace of Murder", Columbo immediately identifies the murderer, but doesn't realize until late that his colleague and confidante on the case, a crime scene investigator, was in on it.
  • Community: While not murder (it's just treated as it) or explicitly stated, the possibility is raised in "Basic Intergluteal Numismatics" — the caught Ass Crack Bandit (who drops quarters down people's ass cracks) turns out to be innocent of that crime, and the ending montage implies it could be one of several recurring characters, some of them having been involved with the investigation. The series finale virtually confirms it, pointing to Annie.
  • Detective Vega on CSI turned out to be one much to the chagrin of many fans.
  • Marty Pino on CSI: NY was a coroner mole variant, processing corpses to extract drugs from organs, again to a lot of negative fan reaction.
  • In the Season 1 finale of Damages, the man who killed the protagonist's fiancee is seen returning to the crime scene to recover a crucial piece of evidence left behind by his accomplice. As he snatches the evidence up and pockets it, one of the cops investigating the case bursts in on him... and greets him as a fellow detective, introduces him to the protagonist, and offers to show him around the crime scene.
  • Death in Paradise: In the very first case in the series, a policewoman murders her boss.
  • In the first-season Dexter episode "Return to Sender", Dexter gets called to the scene of one of his own murders.
    • And again at the beginning of the third season where Dexter takes someone through how their brother was killed. Beforehand he had the opportunity to pick up some evidence he dropped.
    • In fact he was called in when someone mistakenly tried to copy Dexter and ended up in several trash bags. Actually looking over his own crime scenes happens a lot.
    • Oh yeah, and then there's the entire second season, which Miami Metro PD and the FBI spent looking for the Bay Harbor Butcher and Dexter spent evading them.
  • The Doctor Blake Mysteries: In "The Visible World", the murderer is Inspector Llewellyn Sullivan of Special Branch. He claims to have come to Ballarat to supervise the investigation because of the political sensitivities surrounding the case. He was actually in town to kill Alderton and Hannam to tie up loose ends.
  • Eigen Kweek: Piet Despriet, Griet's father, is this, as he's responsible for framing the Welvaert's for weed growing on behalf of a crime syndicate (to sideline Chantal, who's close with the Welvaert's), while also being the magistrate tasked with the investigation of the case. In the end, he has a change of heart and helps out the family (by taking the blame).
  • Ellery Queen: The killer of "The Adventure of the Chinese Dog" turns out to be the sheriff in charge of the murder investigation.
  • Father Brown: In "The Standing Stones", the local police sergeant who is aiding the investigation turns out to be the leader of the cult responsible for the murder.
  • In the second season of Forbrydelsen, Lund's new sidekick Strange is the killer.
  • Forever (2014): Detective Dunn in "Diamonds Are Forever."
  • In an episode of Foyle's War, the killer of the second victim is the detective who's taken over the case Foyle was working on, Foyle himself having been unexpectedly suspended on a flimsy charge that comes down to a case of mistaken identity. It turns out that the other detective set him up for a chance to get at his own victim (It's Personal), who lives near where the investigation is taking place. Foyle's efforts to clear his own name and get back to work conveniently lead to a "Eureka!" Moment about the other detective.
  • Furuhata Ninzaburo:
    • Furuhata makes his superior in "The Final Greeting" believe he is this. In fact, Furuhata knew he was the killer from the beginning.
    • The terrorists in "The Most Dangerous Game" pretend to be detectives trying to catch the terrorists.
    • The bomber in "Red or Blue" is an electronics student who gets coincidentally pressed into replacing an injured bomb investigator. Unlike the previous examples, this wasn't part of his plan and actually interferes with it, since being in investigation headquarters keeps him from making further threatening phone calls as he'd promised he would, putting Furuhata on is trail.
  • In Granite Flats, CIA agent Jim takes over Project Madman under the pretense he's looking for KGB moles. Guess who was the mole?
  • DS Wadsworth is season 2 of Happy Valley, who ends up being on the team investigating the death of the woman he killed.
  • Methos from Highlander, as he's in charge of the division of Watchers who are trying to find him.
  • Memorably used in the pilot episode of Hustle, in which the lead investigator uncovering the crew's confidence schemes is actually a fellow conman, who vanishes with all the cops' evidence before his ruse is uncovered.
  • Bloom in the final episode of Identity.
  • Jake and the Fatman: In "You Turned the Tables on Me", the prosecutor appointed to head the organized crime unit turns out to be literally and figuratively in bed with the biggest mobster in town.
  • Jonathan Creek: Several times. In "Mother Redcap", a police constable is revealed to have killed a judge she was supposed to protect (through a complicated scheme involving an electrified alarm clock and stabbing his body afterwards so everyone would think that the stabbing was the cause of death). And in "Coonskin Cap", a detective uses the apparent serial killer in town to kill his colleague and blame the killer.
  • In 'Jordskott, Göran Wass is implied to be working with the kidnappers. It is later revealed that he is part of a secret organisation that aims to preserve the lives of the few non-human creatures left in Sweden.
  • On Justified, US Marshal Raylan Givens tends to run into those types of moles. In season one, Sheriff Hunter Mosley turns out to be working for a Miami drug cartel and is called in to investigate when his relative botches a hit on Raylan. In season 2 it is an Open Secret that the Benetts are the biggest marijuana growers in the state and Sheriff Doyle Bennett regularly investigates and covers up crimes committed by his brothers. In season 4, the FBI agent in charge of investigating the Tonin crime family has been on their payroll for years. When Raylan goes looking for fugitive Drew Thompson who disappeared in Harlan County 30 years ago, he is helped by new sheriff Shelby Parlow who not only is reluctantly working for crime boss Boyd Crowder but also is Drew Thompson.
  • In Kojak's second episode, "Web of Death," a police detective murders his wife's lover and gets himself assigned to investigate the case. Unluckily for him, his usual partner comes down with appendicitis and he gets paired up with Kojak, who quickly sniffs out the truth.
  • Law & Order: Criminal Intent:
    • In "Major Case", Detective Nichols correctly surmises that the esteemed head of the crime lab killed a teenage drug dealer and is planting evidence to frame another man, but has a hard time convincing captain Ross to go along with him.
    • In "Broad Channel", Nichols and Stevens investigate the murder of a Dirty Cop in an isolated island community. They are forced to work alongside a detective from the Dirty Cop's home precinct. This detective turns out to be a Dirty Cop himself and the one who orchestrated the murder.
  • In the Leverage universe, the actual, real-world never-solved mystery of D.B. Cooper was never solved because... basically right after landing, D.B. Cooper got himself into his own investigation, tampered with the evidence, and then spent the rest of his life as a completely legitimate FBI agent.
  • As you'd expect for a show about a team investigating police corruption this happens repeatedly in Line of Duty. The most notable example is probably DI Cotton, a corrupt police officer who becomes a senior member of anti-corruption squad AC12.
  • Liquidation: The Academic a.k.a. Major Vitaly Krechetov. Also one of his minor associates, Luzhov, a soldier.
  • Agnew and Geddes spend many episodes of Low Winter Sun investigating the murder they committed.
  • This happens in a MADtv (1995) sketch about what it would be like if psychic detectives were real. The medium examines the scene, declares it a perfect murder with no clues, gets a psychic impression, and immediately and easily divines that the investigator did it. Busted.
  • Part of the suspense in series 1 of Marcella is due to Marcella's suspicion that she murdered Grace during one of her blackouts. As a result, she tampers with evidence to hide her involvement...even though it turns out that Henry killed Grace and Marcella merely moved the body after waking up and finding Grace dead.
  • Midsomer Murders has one where a large sum of money goes missing, with two Jerkass "special ops" types are sent to help. Troy immediately sucks up to them, despite their contempt. In the end, it turns out they were in on the heist, with their own boss masterminding (and having done the murders, which wasn't part of the plan).
    • Sgt. Trevor Gibson in "Sleeper Under the Hill" turns out to be involved in the killings and does his best to throw Barnaby and Jones off the trail. He ultimately falls victim to his partner in crime.
  • There was an episode of Monk, "Mr. Monk and the Captain's Marriage", where the killer turned out to be a cop who claims to be sleeping with Stottlemeyer's wife. (He'd lost a tooth down a grate in a fight with the victim, and he needed Stottlemeyer to punch him in the face so that when the tooth was found at the crime scene it wouldn't incriminate him.)
  • In The Mrs Bradley Mysteries, the killer in the final episode turns out to be Inspector Henry Christmas.
  • Murder, She Wrote:
    • In "Murder at the Oasis", Jessica realizes that the killer was Sgt. Barnes, the policeman with whom she has been investigating. He was a hitman who got himself transferred into a small community police force to get at his target.
    • "The Corpse Flew First Class": Inspector Pogson turns out to have been in cahoots with Mr. Hardwick to steal the Empress Carlotta necklace. Hardwick, who had skill in pickpocketing, was to steal the necklace. If the theft came to light, Pogson would smuggle Hardwick and the necklace through airport security by claiming he was a prisoner and the necklace evidence.
    • "Unfinished Business": The killer turns out to be Lt. Kale, who killed Lowell Dixon because he was about to dismiss him for some favors he did for the wrong people and Mr. Roberts because he was blackmailing him.
    • In "The Monte Carlo Murders", the murderer is revealed to be the police inspector Jessica has been assisting in the investigation.
  • The Mystery Files of Shelby Woo': Detective Waterman in "The Alley Cat"
  • A major ongoing plot in the second season of NCIS: New Orleans involves a search for The Mole inside the Department of Homeland Security. The Mole turns out to be Agent Russo, the DHS agent placed in charge of the investigation.
  • In season 3 of Nip/Tuck, serial killer The Carver turns out to be Quentin Costa and Detective Kit McGraw, who's investigating the case, is his sister and accomplice.
  • Over My Dead Body (1990) has everyone impressed by a dashing detective aiding on a case. He takes one look at a suspect's photograph and is immediately able to tell how he pulled off the crime as well as details on his background. An aide on the case gushes to her boss, a Mystery Writer Detective, who immediately says a Sherlock Scan only works in fiction, no one in real life can actually do that, meaning the only way the guy can know all those "facts" is if he's the real killer setting up a fall guy.
  • Perry Mason (2020): Detective Ennis, one of the two LAPD cops on the Season 1 kidnapping-murder case, is one of the kidnappers. He is also the murderer of the other three kidnappers, which he pins on fellow kidnapper George Gannon.
  • '''Person of Interest':
    • In "Proteus", the Serial Killer killed the FBI agent tracking him down and took his place.
    • In "Brotherhood", the DEA Agent working with "Riley" (Reese) mentioned that Dominic had people everywhere - including the DEA. She should know - it's her.
  • Poirot: In thes adaptation of "Murder on the Orient Express", Dr. Constantine's involvement in the murder plot makes his subsequent assistance to Poirot this.
  • Professor T.:
    • DCI Serge Lauwers, who's in league with drug lord Mark Desmedt.
    • DCI Rabet pretended to be one to infiltrate Mark Desmedt's drugs network.
  • In the Psych episode "Psy vs. Psy", Shawn competes with a rival psychic who turns out to be getting her information about a robbery from having been in on the crime. When Shawn puts the amount of stolen money much higher than she does, she realizes her partner is going to double-cross her, kills him, and shows up to investigate the murder. At the crime scene, she touches the body while "psychically reading" it to explain the presence of her DNA.
  • The third episode of Pushing Daisies has Ned called in to investigate the "accidental involuntary manslaughter" he was responsible for in the first episode.
  • Recht Op Recht: Johnny, Stanny's partner turns out to be this in the episode "Vuil". When his commissioner starts to suspect something, he even tries to deflect suspicion by framing Stanny. He subsequently "helps" Hugo solve the case.
  • Several episodes of Running Man set up a detectives-have-to-catch-the-thieves scenario with the latter trying to evade elimination. Of course, the producers don't say that they had to be different people. Such as in Episode 381.
  • In an episode of Sabrina the Teenage Witch, the cast took part in a magical murder mystery party, with Sabrina acting as the detective. Much to her own surprise, she turned out to have committed the crime at the end of the episode.
  • The detectives on the "Strike Team" on The Shield were made to investigate the robbery of a money laundering operation which they were responsible for.
  • In Shots Fired, Platt personally leads the investigation to find Breeland's killer... who happens to be him.
  • Sneaky Pete: In season 2, Taylor is assigned to help investigate the crime he and his grandmother committed.
  • Star Trek: The Next Generation: In "The Mind's Eye", Geordi is assigned to investigate a tampered cargo transporter that was used to beam Federation rifles to Krios. The person who modified then wiped the transporter is himself, but he doesn't remember it.
  • In an episode of Taggart, the initial murders were committed by Detective Chief Superintendent Gordon, and unrelated to the main case.
  • In the Thriller adaptation of Robert Bloch's "Yours Truly, Jack the Ripper," the Ripper is actually Dr. John Carmody, the psychiatrist consulting for the police investigation of the latest string of murders.
  • The Twilight Zone (1985): In "Red Snow", local party official Polvin accompanies KGB colonel Ilyanov during his investigation and has assembled lots of evidence for him, but is one of the vampires who killed the local communists.
  • Vera: In "Old Wounds", Vera investigates a cold case when skeletal remains are found in the woods. The murderer turns out to have been the supervising officer on the original missing person case, who encouraged the investigating officer to do a slipshod job and write the case off as a runaway.
  • Whodunnit? (2013): It's assumed the Killer is sabotaging the investigation. Downplayed, however, as everyone is sabotaging someone else's investigation to avoid being the next victim.
  • Played with in the 5th season of The Wire. Two detectives fake a series of murders of homeless men using bodies recently deceased of natural causes, and end up investigating their own staged crime scenes. Instead of attempting to cover up their work, the aim is to bring publicity to the "killings" in order to restore police funding.
    • Also, an unscrupulous reporter fakes having received a phone call from the imaginary killer in order to further his own career, much to the "mastermind's" surprise; who then decides to call the reporter for real as the killer to bring in even more publicity.
  • Witse:
    • The two-parter "Gif" had crime lab technician and Mr. Exposition Robert (Bob) Daniëls as the killer. In the same episode, another inspector, Linda de Maeschalk, was suspected of being this by Witse, but she turned out to be a Red Herring Mole in the end.
    • In the episode "Ijskoud", the victims were murdered by the coroner who examined the bodies.
    • In the two-parter "Achter de waarheid", Witse's team gets help from Rik Cuppens, a DCI from the missing persons squad. When Witse and his team unravel a human trafficking ring, Rik is part of the arrest team responsible for the arrest of the last remaining organizer of this ring. However, a gunfight takes place throughout the arrest and Rik shoots the organizer in self-defence. Or, at least, that's what he wants his colleagues to think. In reality, he was in on the human trafficking ring and even delivered wayward girls to them. When the net started to close, he shot the organizer (and last remaining witness) to conceal his involvement in the crime.

  • In the Vocaloid song "The Riddle Solver Who Can't Solve Riddles", the detective (played by Len) initially appears to be unmasking the girl (played by Rin) as the culprit of the murders. However, in the end, the detective reveals himself to be the actual killer.

    Tabletop Games 
  • The board game Clue, possibly (and definitely if you're playing with six players).
    • Hilarious in the SNES version, where a cutscene will play out where the winner deduces the killer and the killer gets taken away by the cops while cursing the winner. "Prof. Plum: The killer was Prof. Plum. [...] Curse you, Prof. Plum!". Also, that apparently still counts as a win.
    • Mr. Green: I won, I won! I may be going to jail, but I won!
  • Deception: Murder In Hong Kong is a Social Deduction Game, where various detectives try solving a murder based on clues given by the Forensic Scientist, but one or more of the detectives was part of the murder. The expansion pack Undercover Allies also adds an event that can cause the Forensic Scientist to switch teams.

  • The Bat reveals in the end that the man who introduced himself to Cornelia in the first act as the detective Mr. Anderson is actually the Bat, who beat up the real Mr. Anderson and tied him in the garage.
  • The Broken Jug (Der zerbrochne Krug) by Heinrich von Kleist is about a judge who is to sit in judgement on a crime he committed himself.
  • The Play That Goes Wrong: Inspector Carter is revealed to be this in the Play Within a Play The Murder at Haversham Manor. Of course, by the time this revelation is made, absolutely no one cares about the plot of the play anymore. It doesn't help that, due to various disasters and some incompetence, this reveal is made two times before it's supposed to be.

    Video Games 
  • Later on in Disco Elysium, the amnesiac protagonist is presented with the possibility that he might have been a corrupt cop under the mob's payroll who's been using his position to act as a hitman. The question is only answered at the very end by your original partner, who bluntly states that no mob boss would ever use you in that capacity due to your mental instability.
  • In Fallout: New Vegas, you can assist in an investigation regarding a possible Legion mole within Camp McCarran, the main base of the NCR. You'll eventually learn that the mole is none other than Captain Ronald Curtis, the very person who assigned you to find the mole and a Legion Frumentarii.
  • Scott Shelby (the Origami Killer) from Heavy Rain, who is pretending to be a private eye hired by the victims' families so he can get access to and dispose of any evidence that implicates him.
  • In Judgment, the killer known as "The Mole" note  that Yagami spends the entire game pursuing is none other than Detective Mitsuru Kuroiwa, the detective assigned to the case.
  • A sub-plot in the CSI game Fatal Conspiracy involves a drug cartel, being investigated by one FBI Agent Huntby. In the second to last case, it's revealed that Huntby was working for the cartel.
  • Tohru Adachi in Persona 4 turns out to be responsible for the serial kidnappings, two murders, and seven attempted murders.
  • Goro Akechi in Persona 5 turns out to be responsible for the mental shutdowns and psychotic breakdowns, and is The Dragon to Big Bad Masayoshi Shido. Apparently in the Persona series, you can never trust a detective whose name is A—chi.
  • In The Witcher 3: Wild Hunt, the sidequest "Carnal Sins" consists in investigating the crimes of a Se7en-esque Serial Killer, who is eventually revealed to be Hubert Rejk, the local coroner.

    Visual Novels 
  • Ace Attorney:
    • The first game:
      • Manfred von Karma ends up prosecuting Miles Edgeworth for the DL-6 Incident, the murder of Edgeworth's father which von Karma himself carried out. Manfred was also The Man Behind the Man to the culprit of the Gourd Lake murder, instructing him to frame Edgeworth specifically to engineer circumstances that would prompt Edgeworth to confess his supposed guilt for DL-6.
      • Damon Gant, back when he wasn't the chief of police yet, was involved in the SL-9 Incident, first as an investigator of its primary suspect Joe Darke, then as the culprit of its final murder. He killed Neil Marshall, framed Ema Skye for it to her sister Lana Skye, and then used that to blackmail Lana into framing Darke for the death of Marshall.
    • Trials and Tribulations:
      • Luke Atmey isn't actually the thief he was looking for, but he's the mastermind behind his heists.
      • Furio Tigre was Maggey Byrde's defense attorney and the culprit of the crime she was being tried for. In fact, he intentionally defended her badly so she would be declared guilty for his crime.
    • Ace Attorney Investigations:
      • Tyrell Badd, one leg of the Yatagarasu, who made sure to be the detective assigned to the Yatagarasu case specifically to destroy any evidence his partners left behind. The other two members are Byrne Faraday (a prosecutor) and Calisto Yew (a defense attorney).
      • Prosecutor Jacques Portsman is a member of the international smuggling ring that Interpol is investigating.
      • Prosecutor Bansai Ichiyanagi/Blaise Debeste killed Jill Crane and pretended to be unrelated to the crime.
    • Daryan Crescend from Interpol killed another Interpol agent and came up with an alibi to disconnect himself from the investigation.
    • Detective Bobby Fulbright (or rather, a spy who killed the real Fullbright and took his place) investigates the murder of Clay Terran while being the culprit himself. He also killed Metis Cykes several years earlier, another murder you have to solve.
  • Danganronpa V3: Killing Harmony:
  • Defied in The Murder of Sonic the Hedgehog; one of the rules of Amy's murder mystery party is that Tails, as the detective, can't be the murderer.

    Web Original 
  • In Doom House, Officer Cop is initially introduced as the dispatched police officer responding to Reginald P. Linux's 911 call, but he turns out to be the one responsible for the horror Linux is experiencing inside the doom house.
  • Played for laughs in the SMBC Theater short Homicide Detective:
    "Why, I've been the killer all along [...] But how?"
    "Remember when you stabbed this guy... five minutes ago?"

    Western Animation 
  • Dan Vs.: In the episode "Dan Vs. Elise's Parents", Dan tries to get Elise's father arrested by manufacturing evidence that he's the head of the mafia. This evidence is enough to convince the police to open their own investigation. The detective they send turns out to be a mole, working for the real mafia, whom Elise's mother is the head of.
  • The Legend of Korra: The city of Zoufu is infiltrated, and Suyin suspects collaborators within the security forces, so Korra and her friends interrogate the guards. They are assisted by Suyin's trusted advisor Aiwei who, thanks to his bending ability, is a Living Lie Detector. As the investigation goes nowhere, they come to the uncomfortable realization that Aiwei is one of the only people in the city who can keep a secret.
  • Voltron: Legendary Defender: Zarkon gives the task of finding the traitor on his flagship to Thace, who is the Blade of Marmora's agent on the ship.

    Real Life 
  • Dr. Harold Shipman wrote the death certificates for his victims, to make the deaths out to be natural.
  • FBI agent Robert Hanssen, who sent enormous amounts of intelligence to the Soviet Union and later Russian Federation during a career as a spy that lasted over twenty years, was at one time head of a task force charged with ferreting out the suspected mole in the FBI—himself. The film Breach is based on this story.
  • FBI agent John Connolly was involved in helping mobster James J. "Whitey" Bulger get away with his crimes for as long as he did. Colin Sullivan in The Departed was loosely based on him, while Frank Costello from the same movie is based on Bulger. Black Mass is a more direct treatment of Connolly and Bulger's partnership. Connolly is currently in a Massachusetts prison for a 40-year sentence after serving a 10-year sentence in federal prison. It was partially thanks to him that Bulger was able to escape as a fugitive for as long as he did.
  • Kim Philby was head of the counter-espionage section MI6 for two years, while all the time he was a spy for the Soviet Union, and thus charged with stopping himself before he defected.
    • Philby and his colleagues blew the cover of several British intelligence officers, one of whom would go on to write spy novels under the name John le Carré. Tinker, Tailor, Soldier, Spy is a fictionalized version of the whole affair.
  • CIA agent Aldrich Ames was tasked with attempting to make contacts with Soviet double agents, then later assigned to the counterintelligence section, searching for Soviet moles. Instead, he became a double agent for the Soviets (not for ideology, but because he "wanted a nicer car"), passing along information on American-controlled double agents, many of whom were subsequently executed. Not only that, but he blew the cover of a high-ranking KGB agent, Oleg Gordievsky, who was secretly working for the British, who barely managed to escape across the Finnish border in the trunk of a car, and who still lives in hiding under an assumed name.
  • Dennis Rader, the B.T.K. Killer, took a job for the security firm ADT Security Services shortly after beginning his series of murders. Part of his job was installing home alarms, and they were sometimes touted as helping protect people from B.T.K. He once reported one of his own murders to the police from a phone box just a few blocks from his offices. In addition, during this time he was studying for a degree in Criminal Justice- he wanted a thorough knowledge of law enforcement procedure.
  • In 1978-1979 in France, a series of various felonies (five assaults, one murder, car thefts...) in the same region was eventually attributed to Alain Lamare, a gendarme who was part of the persons who investigated this crime spree. Lamare was diagnosed schizophrenic and declared non-responsible, so he was never tried.
  • Non-fiction essay Jack L'Eventreur démasquénote  by French writer Sophie Herfort postulates Jack the Ripper was Melville Macnaghten, one of the Scotland Yard investigators who worked on the cases. Among other clues, Herfort points out Macnaghten's hatred of prostitutes, his own skills in taxidermy (i.e. Macnaghten actually had the skills needed to disembowel the victims), and his conflicted relation with police chief Charles Warren, the latter clue being his direct motive. According to this theory, Macnaghten's series of gruesome crimes was supposed to raise a ruckus to harm Warren's career, and indeed the murder series stopped once Warren resigned.