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).
But it gets worse. The Israeli scientists were actually only announcing a test that they had developed to tell whether a DNA sample had been fabricated. It seems that the fabrication process itself is so simple that any grad student with access to a lab could do it. Which leads to this question: Have they been doing it? How do we know that the DNA samples that our justice system has come to rely on so heavily haven't been manipulated multiple times over the years? Perhaps, as the leader of the human identity testing project at the National Institute of Standards and Technology noted, the "average criminal" isn't going to be able to do something like that. But what about the criminal with brains or means? What about the detective in the scenario above who is desperate to build an air-tight case against a sadistic killer? Or a defense lawyer looking for "new" evidence to clear her client?