>> reporter: all of that is so counter intuitive and so bizarre that, you know, you wouldn't be expected to believe such a thing. >> no. >> reporter: it was stunning. really. liz apparently impersonating cari for years sending thousands of texts and emails in her name. but now they had a bigger question and a much bigger problem. >> i guess part of the worry was that if-- even if we could prove that it's liz sending all this stuff out as cari, well, that doesn't prove murder. >> reporter: murder? yes. sgt. doty and cpl. avis believed that liz killed cari out of jealousy, impersonated cari in order to win dave back, then tried to frame his ex-partner amy for everything. even going so far as to set her own house on fire, kill the family pets and shoot herself in the leg. pretty wild stuff. but, could they prove it? >> we did s-- need something more. so, we still weren't quite sure how to get to that point. and then liz herself, by accusing amy of shooting her gave them their big idea. >> and that's when we introduced