Every now and then, although much less recently, a reporter asks me the inane question, “Is DNA a silver bullet?” While the obvious answer is “no,” the follow-up is always, “DNA will never replace a thoughtful, creative detective with the proper resources.” No one has ever disagreed with me on that. That is however, the positive spin on the answer. Another approach is, “Regardless of how good the technology is, it is ultimately a system run by human beings—mistakes can and will be made.” Does the case of a certain Heisman trophy winner come to mind?

Last fall, the British newspaper The Guardian ran a lengthy article on the most significant serial killer in Europe. The killer, dubbed “the woman without a face” was implicated in six murders, including the murder of a female German police officer shot in the back of the head while she sat in her patrol car. The killer was not just a brutal executioner though, she was a thief (DNA was found at numerous burglary scenes) and a drug addict (DNA was found on a heroin syringe). She was a frequent traveler, her DNA being found at crime scenes in Germany, Austria, and France. And her weapon of choice varied (DNA was found on a stone used to smash a victim’s face).

The “woman without a face” was law enforcement’s worst nightmare—elusive, seemingly indiscriminant, violent, and well traveled. DNA samples were taken from 3,000 homeless women believed to be drug users. A reward of €100,000 ($135,000) was posted. And German, Austrian, and French authorities spent millions of Euros on the investigation.

In the course of the story, the reporter interviewed a prosecutor (note this isn’t just about the police) in charge of the investigation. When talking about the status of the unsolved case, the prosecutor said, “There are still no witnesses, and no other evidence. All of us on the various teams talk to each other two or three times a month. We meet, we e-mail, but mostly we wait for another report saying the same DNA has turned up.”

Uh-oh. That’s a bad statement.

There is too much art in criminal investigation to call it a scientific endeavor. At the same time, we rely too much on science now to ignore the scientific method that requires us to always question our assumptions. If we are waiting “for another report saying the same DNA has turned up,” we have lost sight of our need to rigorously apply the scientific method to the context of our criminal investigations.

As it turns out, our serial killer, our female executioner without a face, is really a 71-year-old woman armed with cotton swabs and a poor protocol for avoiding contamination. Police announced in March that they had determined cotton swabs were contaminated with the DNA of the woman packaging them. The tip off? Police were attempting to identify the body of a burn victim they believed to be an asylum seeker. They found the victim’s application for asylum and swabbed the fingerprints it contained. The DNA analysis yielded the profile of the notorious serial killer. There was only one problem—the body they were seeking to identify, and to whom the fingerprints belonged, was male. Swabbing the prints again with a different swab kit, the analysis yielded a different profile, and appropriately, the profile of a male. Thus began an embarrassing exercise in hindsight.

Let’s start with the basics. Female serial killers like this are rare. Serial killers with this diversity of crimes are rare: police executioner, common thief, and school burglar. Some of the crimes were committed with “accomplices” who were convicted of those crimes but who denied the existence of a female cohort. There was no connection between the criminals she allegedly commited her various and diverse crimes with. Throughout this entire crime spree, no one had ever seen her. And in those instances where the vaguest of descriptions could be given, it was of someone who looked like a man. And that rock used to smash a victim’s face? No blood, no tissue, just the “perpetrator’s” DNA.

To be clear, the Europeans are no slouches at using DNA to solve crime. This is just one reminder, one lesson in a long line of examples, that compels all of us to be more rigorous in questioning our investigative assumptions. As bad as the wasted money is, as embarrassing as the failures are, they pale in seriousness and severity to the realization that “over reliance” on DNA—and a failure to question more and assume less—means that several killers remain on the loose.

Chris Asplen is a former Assistant U.S. Attorney and local prosecutor specializing in the prosecution of sex crime and child abuse.He was also formerly the Executive Director of the National Commission on the Future of DNA Evidence for the U.S. Department of Justice and Director of the DNA Unit for the National District Attorneys Association. Currently, he consults with local, state, and foreign governments and law enforcement agencies on the use of forensic DNA technology. Chris is also a member of the Crime Victim Bar Association.