disappearance, he and his partner brought avery in for an interview. "dateline" filed a freedom of information request to obtain the video. >> you know the key is there because you put the key there. that's the only way the key gets there. >> no. >> yes, steve, yes. >> avery was defiant and even claimed cops were framing him, planting evidence. >> the cops have evidence. >> fassbender and wiegert weren't buying it and placed him under arrest. >> i didn't do it. somebody's doing a good job on me. >> investigators were convinced they had their man. still, they didn't know how or why teresa had been killed. coming up -- shackles, handcuffs, strange behavior. >> teresa's creeped out. >> could something in his past have led him to murder? when "dateline" continues. nexgard chew comes the confidence, you're doing what's right, to protect your dog from fleas and ticks for a full month. it's the #1 vet recommended protection.