salvage yard to take photos, some at steven avery's request. >> he starts making calls directly to teresa halbach>> and kratz determined on the day before one of these visits, avery purchased these shackles and handcuffs. >> when teresa arrives -- >> he answers the door wearing a small white towel. and teresa, we know is creeped out by that behavior. she tells friends and co-workers about that. >> kratz believes avery's behavior shows signs of a sexual obsession with teresa that led to murder. >> why would steven avery do this when he was on the verge of getting this big windfall, potentially millions of dollars? >> that's a great question. you know, i don't know. we know he's a psychopath. we know that he has -- >> do we know that? >> well, i -- let's say, i allege that. i believe that, deep down. >> kratz discovered more in avery's past. 18 years before teresa's murder, avery wrote this letter to his estranged wife from prison, saying, i will kill you. and a former fiancee told police he had physically abused her. and kratz said he learned something even more disturbing. this 2006 police report