They call her “the woman without a face”, a serial killer who has stalked Western Europe for more than 15 years, murdering young and old while leaving no witness who could describe her. The Phantom of Heilbronn, as she has also become known – an allusion to the scene of her most notorious crime – has apparently killed, mugged and burgled across Germany, France and Austria, leaving her DNA at 40 crime scenes.

Luckless German detectives have travelled thousands of miles in pursuit, perplexed as to how she could evade capture for so long. A €300,000 (£280,500) bounty has been put on her head.

Now it seems the Phantom might not, in fact, exist: officers believe that her “crime spree” was a fiction created by contaminated cotton swabs.

The Federal Criminal Police Office (FCPO) is investigating the theory amid strong suspicions that the female DNA evidence tracing her career comes from an employee involved in making the swabs or packing the cotton buds. If these fears are confirmed, it would throw the investigations of six murders and multiple burglaries into disarray and, as the head of the Baden-Württemberg state police union put it, be “a very embarrassing story”.

Detectives have been on the trail of the Phantom since 2001, when a seven-year-old boy stepped on a heroin syringe in the German town of Gerolstein. His worried parents insisted on a full blood analysis, and when DNA traces were fed into a central databank a match was found to two unsolved murders.

The first was that of a 62-year-old woman who was strangled in 1993. The culprit had apparently left her DNA on a teacup. Fourteen years after that death, in 2007, the Phantom’s notoriety peaked when it appeared that she had been sitting in the back seat of a police car in Heilbronn, southwest Germany, as a 22-year-old policewoman was shot from behind.

Doubts about the Phantom’s reality were raised when her DNA was found on the burnt body of an asylum-seeker in France. “Obviously that was impossible, as the asylum-seeker was a man and the Phantom’s DNA belonged to a woman,” Ernst Meiners, spokesman for the public prosecutor’s office in Saarbrücken, west Germany, said. A second examination failed to find the Phantom’s DNA on the body’s finger-prints. “That aroused suspicions that the materials were contaminated.”

Forensic science experts in Stuttgart have analysed hundreds of unused cotton swabs since fears were first raised in April last year, although no source of contamination has been found. Officials confirmed that the FCPO had been investigating since last month, and the magazine Stern reported that DNA samples had been taken from women staff at a medical supplies company. Police in the Austrian city of Linz have also announced that they are investigating, after developing their own doubts about the existence of the Phantom.

If proved true, this latest development would explain why the killer has evaded capture for so long, despite leaving DNA on guns, cigarette packets and even nibbled biscuits at crime scenes. It would explain why she has alternated between pilfering from sheds and cold-blooded murder, and account for her wide-ranging travels.

It would also, perhaps, shed light on the reasons why police last year released a photofit of a male suspect after witnesses described a man they had seen at one of the crime scenes, leading to speculation that the killer could be a transsexual.

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