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Pressed sam for more details. >> the clothing you were wearing saturday? >> i think it was a guess sweatshirt. >> reporter: sam readily gave those clothes to the police. he let them search his house too. and he willingly gave samples of his hair and blood. but something about sam's behavior the week cathy was missing bothered her family. mary said she'd repeatedly paged sam looking for cathy, but he was slow to respond. he call you back? >> not at first. >> reporter: but eventually he did? >> eventually, yes. >> reporter: and he said what, he hadn't seen her? >> he hadn't seen her. >> reporter...

Friends, because technology changes. >> reporter: at the time of the murder, dna tests could only be done on big samples like blood spatter. but in the years since, analysis became possible for touch-dna, the microscopic calling cards many of us leave behind just by putting our hands on something. if sam and javier did place cathy's body in the car's trunk, maybe, thought larry, there would be touch-dna on her clothes. >> search for dna in the areas that might be grabbed. especially areas that might be grabbed that don't have blood on them, from cathy. >> so you looked, what, on the ankles and...

Armpit." >> reporter: "did we tell you?" no, we didn't tell you. how long had they known that? >> a couple of months, at least. >> and they, what, forgot to call? >> i think the examiner who had been doing it was waiting for additional results and didn't realize that she hadn't notified us. >> so the idea of touch-dna paid off? >> it did. >> reporter: but not quite the way they had all hoped. unbelievable that you would get touch-dna evidence back that long after the fact. >> right. >> but bad news, it doesn't have sam lopez's name on it. >> right. that's right. >> reporter: so the strongest p...

I'd rather remember the loving times we had together. and they're not going to take that away from me. >> that's all for this edition of dateline extra. i'm craig melvin. thank you for watching. she was a mom looking for her baby, and her baby hadn't come home. >> reporter: cathy's week had been nothing but trouble. >> she couldn't tell me what happened. >> she was so upset. >> reporter: it was about to get much worse. >> my mom would say "if you find the car, you'll find cathy." >> you didn't want it to be dark because you wanted to keep looking. >> she was stabbed multiple times. there was...

This saturday that i'm not gonna take off with him, that i will not elope." that was wednesday. and then saturday she never came home. >> reporter: sam's admitted jealousy, cathy's doting on albert, a rejected proposal. all of it seemed to add up to motive. but motives don't prove murder. there was still no physical evidence tying sam to the crime. all the blood on the car had been tested, and it was all cathy's. but then, while combing the case file, detective daron wyatt learned something shocking. there were blood and hair samples that had never been sent to the crime lab. and when those sa...