Researching a novel - q about a single key witness to murder?

I’m trying to plot out a crime and its resolution for a novel, so drama is key here, but I also want the situation to feel plausible and authentic. I’m looking for one of those “aha” moments during which a single key witness comes forward and provides the final puzzle piece, proof that the murderer is guilty. (If this sounds like a bad episode of Law & Order then that’s actually good; I can make it work.) A single eyewitness is not strong enough. She has to provide a detail only the investigators would know, and something the murderer can’t deny. And one more challenge…this needs to occur seven years after the murder. I’ve been reading real cases until my eyes glaze over. Would love to hear your thoughts. Thanks!