>> reporter: back in 2002, at 18, sam herr was charged in the murder of another teenager. he was acquitted. but sam's dad knew that in hindsight, it didn't look good. that doesn't buy you a lot of rhythm from law enforcement? >> no, and i understood that. that's why it was imperative to start looking immediately. >> reporter: steve wanted to find his son, to prove he was innocent. orange county prosecutor matt murphy also wanted to find sam quickly for entirely different reasons. >> you got a dead woman in his apartment. yet he's missing. and then you factor in afghanistan. and you start thinking, "okay, he's -- he's a trained killer. he'd been accused of this before." and you think, "okay, this -- this guy is potentially very dangerous." >> reporter: the detectives and the dad hunted sam independently but on parallel tracks. detectives spoke with sam's neighbors. >> i saw sam that day and had a short conversation with sam. >> reporter: rachel and dan said