What was the "Phantom of Heilbronn", and what does it teach about DNA evidence?

A supposed female serial offender whose DNA turned up at 40 crime scenes. In fact the DNA came from a factory worker who had contaminated the cotton swabs used to collect it. The lesson: a DNA match is only as reliable as the whole chain of collection.

Between 2001 and 2009, the same female DNA was found at 40 crime scenes, most of them in Germany, including the 2007 murder of police officer Michèle Kiesewetter in Heilbronn. Investigators hunted for the "phantom" in a huge, costly investigation without success. They grew suspicious when the same DNA appeared in an Austrian homicide that had already been solved. A test on an unused cotton swab then found the trace: it belonged to a woman working at the factory that packed the swabs.

Why it matters:

• Contamination: the analysis was correct, but the sample was not. DNA's high evidential value hid a mistake earlier in the process.
• Misdirected investigation: years were spent on a non-existent serial offender. In the Kiesewetter case, this kept investigators from finding the real perpetrators, the neo-Nazi terror group NSU (Nationalsozialistischer Untergrund).
• Blind trust in technology: because DNA "can't be wrong", obvious doubts (one woman at very different types of crime) were dismissed for years.

Go deeper:

• Wikipedia: Phantom of Heilbronn — the full story of the contaminated cotton swabs.
• Wikipedia (DE): Mord an Michèle Kiesewetter — the Heilbronn police murder, the phantom and the later link to the NSU (German).