everything in plastic bags to containers. if that turned up in the foam on marshall's boots but not in the foam at the crime scene, wouldn't that exclude marshall as a suspect? the detective protested phthalates must have gotten in the boots because they were stored in a plastic container all those years. but to the scientists, facts are facts, those two pieces of foams did not share the same chemical signature. >> to hear her say one has phthalates and one doesn't, i have to tell you, it was devastating. >> not your best day. >> not the best day. because it was the piece we needed to do something more than we had from 1984. without that connection to the boot, i don't think we have a case. coming up, investigators make one last-ditch efforts to talk to their suspect. they track him down here leading a surprising life when "dateline" continues. "dateline" continues