Detectives Learn A Tough Lesson In DNA Sample Contamination

| by Jonathan Wolfe

Here’s a real life murder mystery that reads like it’s straight out of a forensic thriller. The full story was written by forensic scientist Dr. Mike Silverman and published by the BBC yesterday.

In 1997, Silverman was trying to find out who was responsible for the brutal murder of a young woman in London. DNA forensics was a relatively new field at the time, but Silverman and his team were making use of DNA analysis techniques nonetheless.

After finding a piece of biological matter under the woman’s fingernails, detectives decided to collect a sample and run an analysis. They figured the woman may have scratched her killer while trying to escape, and if so the sample would likely belong to the murderer. This wasn’t the case. In fact, the analysis raised more questions than it answered.

The analyzed DNA belonged to a woman who died a full three weeks prior to the latest victim. Silverman was stumped. How could fresh DNA found on a murder victim belong to someone who died weeks ago? As Silverman would soon find out, he was learning a valuable lesson in DNA contamination.

After looking everywhere for answers, he finally ordered a DNA analysis to be run on the scissors used to collect the sample. The analysis turned up DNA from three separate people, all of whom were recently killed. He had his answer. Despite protocol calling for the scissors to be sterilized between each use, DNA from past victims remained on the blades.

The finding prompted Silverman to have all of his lab’s equipment analyzed. Sure enough, many of the instruments were contaminated with DNA from numerous past victims. Silverman immediately called for disposable scissors and tools to be used when collecting evidence – a policy that is still in place today.

“Modern DNA analysis is now so sensitive that contamination is a major issue, with the potential to send criminal investigations spiralling off in the wrong direction,” Silverman says.

He then goes on to describe the 2007 “Phantom of Heilbronn” case in Germany in which police found the DNA of the same woman at over 40 crime scenes. Police suspected they had a serial killer on their hands, when in fact all they had was a factory worker. A woman who worked at the factory the swabs were made at accidentally contaminated the swabs with her DNA, leading police on a wild goose chase for a completely fictitious killer.