There's no way this scumbag didn't do it. We've got three freaking people who heard them arguing in the apartment twenty minutes before they heard the screams and dialed 911. Now the lab tells me that that there's only the girl's blood on the knife, no DNA from this asshole . . . The door is wide open for him to argue that someone came in after he left and did the stabbing. I will not let him get away with this!
The best thing about DNA evidence is the certainty. Unlike eye-witness testimony, which is frequently unreliable and subject to various psychological motivations, and fingerprint analysis which depends on "expert" interpretation, DNA evidence is a lock. We all feel better when we find out that the killer's DNA is on the gun or under the victim's fingernails, because there's no faking that, right?
Wrong. Someone is always building a better mouse-trap. Scientists in Israel recently demonstrated that it is possible to fabricate DNA evidence (they took blood from a woman, centrifuged it to remove the white cells containing the DNA, and then added DNA that had been amplified from a man's hair - it passed as a legitimate sample at a leading forensics lab right here in the good, old USA).