he starts making calls directly to teresa halbach rather than going through "auto trader." (voiceover): and kratz determined that on the day before one of those visits avery purchased these shackles and handcuffs. and when teresa arrived? ken kratz: he answers the door that day wearing a small white towel, and teresa we know is creeped out by that behavior. she tells friends and co-workers about that. andrea canning (voiceover): kratz believes avery's behavior shows signs of a sexual obsession with teresa that led to murder. andrea canning: 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. i don't know. we know that he's a psychopath. we know that he has-- do we know that? well, i-- let's say i alleged that, ok? i believe that deep down. andrea canning (voiceover): 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. a