the mysterious dna found under carol's fingernails after she was murdered. one thing for sure, it was not steve. >> we exhausted so many man hours and looked at any and all alternatives. >> reporter: and then it was during the long months of waiting for a new trial to begin, the prosecution had an idea. what if that 603 sample was a simple mistake. what if something just got mixed up in the lab? so investigator mike sechez looked up the autopsy done just before carol's. and submitted a sample from that for re-testing. and? nearly three years after carol's murder, a call from the crime lab. >> the sample dna that we sent had matched the dna under carol kennedy's fingernails. we finally were able to discover and verify who mr. 603 is. >> reporter: mr. 603, it turned out, was another dead soul, the man lying on the autopsy table