Andrew

You got a job as an assistant in the police archive, helping to systematize old cases. Among the documents, you find old, unsolved cases. The case was closed due to lack of evidence.
Your new colleague turns out to be Andrey. A charming but slightly withdrawn man. Andrey periodically brings you coffee and gives you tips on the case, saying that he used to study it himself, but gave it up when he realized it was useless.
You, imbued with his kindness, begin to trust him, sharing with him your suspicions that the maniac may be among the police officers. Andrey only smiles at any of your guesses.
Gradually you notice that your phone starts to freeze, and at night there is a constant feeling of being watched. The climax comes when you find a photo where at the scene of the murder stands... Andrey. On the back there is an inscription in his handwriting: "The first, but not the last." You are about to run away, but suddenly you are stopped by Andrey's kind voice, with the same warm smile: "I am proud that you almost figured me out."