. >> reporter: investigators latek confir court documents show matthew was obsessed with the movie "americancho." and that his private instagram account shows him dressed as the main character, a serial killer. also according to court documents, a friend told police that phelps expressed interest in what it would be like to kill someone. lauren's family says they had no idea about his dark social media trail. they shared these photos with us from her last days. she looks happy, but her father says he saw otherwise. >> the last time i see my daughter, it didn't look like lauren at all. she look ted a mess. but -- i regret it now that i didn't ask questions. >> how do you reconcile that regret, not asking her why, what was going on? >> i can't. i can't. i mean -- i think about it every day, what i did wrong. what i missed. >> reporter: two days later, lauren was brutally murdered. her mother recalls the police knocking on her door in the middle of the night. >> and the looks on their faces. i can't get that out of my head. and i thought, did matt kill her? and they asked me, why would you ask