Can DNA at a crime scene point to a killer who does not exist?

Yes, it can, and for years it did. Between 1993 and 2009, the same unidentified woman's DNA turned up at 40 crime scenes in Austria, France and Germany, murders and burglaries alike. She became known as the Phantom of Heilbronn, a presumed female serial killer. In March 2009, investigators concluded she did not exist. The DNA belonged to a woman who worked at the factory that made the cotton swabs used to collect DNA samples.

The clue had been there for a while. Mitochondrial DNA from samples in Austria showed characteristics most often found among people in Eastern Europe and Russia. By 2008, some officers suspected the evidence was misleading and came from contamination. The proof arrived in March 2009: while trying to identify a corpse, investigators found the Phantom's female DNA in fingerprints on a male asylum seeker's application. A reward of 300,000 euros had just been raised in January, for a person who was never there.

How could that happen? The swabs were sterile, but they were not certified for human DNA collection. The contaminated swabs all came from the same factory, which employed several Eastern European women, and the Bavarian police bought swabs from a different factory, so the Phantom's DNA never turned up in Bavaria, even though that state is close to many of the scenes where it was found.

The lesson is simple and a little humbling: a DNA match tells you whose DNA it is, not how it got there. After the case, the International Organization for Standardization published a standard, ISO 18385 in 2016, for collection products free of human DNA contamination.