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Would it qualify if the characters know from the start who did it, but has no evidence and has to gather some?
(Example: a Living Lie Detector who figures in the first five second who has done it by seeing through the lies like we see through a window, and is attempting to find actual evidence since his powers don't count as such.)
Forehead slap moment: this should be called Howdunnit.
(I know, I'm replying three years after the comment, but...) The name "Howdunnit" is already taken: it means a traditional (non-reverse) Whodunnit in which, although the villain's identity will be revealed at the end, the main Driving Question is how it was done: usually an "impossible" crime or Locked Room Mystery.
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How well does it match the trope?