An interesting story from Science and Stuff:
DNA traces of an unknown eastern-European woman had been found at almost 17 crime scenes, including two murders (including a 22 year old police officer) but also car jackings, unprofessional break-ins and on a bullet fired in a marital dispute. The crimes where spread around a large area including south-west Germany, France and Switzerland.
It now turns out that the several-hundred-men task force might have really been chasing a phantom. Alarmed by the apparent randomness of the crimes, involving both highly professional work and seemingly amateur break-ins, they started checking for contaminations in the labwork. The likeliest suspect now are the cotton swabs used to collect evidence at the crime scene. All the swabs used in the forensics works were sourced from the same supplier, a company in northern Germany that employs several eastern-European women that would fit the profile. Even more inciminating, the state of Bavaria lies right in the center of the crimes’ locations, without ever finding matching DNA in crimes on its territory. Guess what: they get their cotton swabs from a different supplier.
While the suspicion had already been growing in the last few months, the smoking gun apparently was a case where they tried to match a burned (male) corpse to DNA collected from fingerprint samples an asylum-seeker had given a few months earlier. The first test showed a match between those fingerprints and the Phantom’s DNA while a second test did not.
By the way: contaminated cotton swabs aren”t as trivial to avoid as one might think. It’s relatively easy to sterilize cotton to prevent infections. Forensics however require a complete destruction or removal of any DNA contamination, which is apparently a lot harder.