set-up for vampire slaying

I have a character who's a vampire detective - not in the sense of a detective for humans who happens to be a vampire, but in the sense of a detective for vampires, who tracks down vampire slayers. Anyway, I'm having him work a case of the death of 800 year old Sonja (recognized by smell as she's turned to dust).

Here are the facts I've decided about the case:

• the killer is a gangster who got started in vampire slaying because he found out an old enemy of his had become a vampire, and he's kept up slaying because a mysterious person introduced him to an organization that'll pay him a certain amount for killing off the vampires they want killed

• he followed Sonja from the blood bank, where she went to pick up some food for the week, and caught her by surprise when she was alone in an alleyway

• he accidentally left a boot print in the ground nearby

Other than that, I don't know the details of the case - who found Sonja's remains, how the guy killed her, etc. (By the way, vampires in my story can be killed by a perfectly-aimed wooden stake, a sufficiently serious wound caused by silver, or exposure to sunlight, but not fire.)

Any suggestions?

Maybe some of the ashes got on the gangster when he killed the vampire? If this detective can make out the scent, he might be able to follow it for a while. Maybe to a place where he can get security cam footage?

Also, not sure about the blood bank thing. Is it run by vampires as well? Because otherwise I'm finding it a little hard to take seriously that a vampire just walked in and got some blood.

One of the guys working at the blood bank secretly sells stuff to vampires - mostly the blood that would be thrown out anyway (eg HIV positive) since vampires don't get human illnesses.