It's the fingerprints on the stuff he may have left behind and the fake IDs he used to book the hostel. I'm not sure on how accurate forensics is for a 3d printed gun.

I think last week as the investigation went on and he was nowhere to be found the general vibe was that he was some sort of criminal mastermind, with possible accomplices supporting him. I think it might have just been dumb luck he evaded capture as long as he did.