For David Butler, it began with a knock on the door early one November morning, seven years ago. When he opened it, officers from the Merseyside police were standing on his doorstep. The retired taxi driver was being arrested for murder.
The police said they had evidence connecting Butler to the death of Anne Marie Foy, a 46-year-old sex worker who had been battered and strangled in Liverpool in 2005.
Butler’s DNA, it turned out, had been logged into the UK national database after a 1998 investigation into a break-in at the home he shared with his mother. A partial match had been made to DNA found on Foy’s fingernail clippings and cardigan buttons. This, combined with CCTV evidence of a distinctive taxi seen near the scene, led the prosecutor to tell the jury in Butler’s trial that the DNA information “provides compelling evidence that the defendant was in contact with Anne Marie Foy at the time immediately before she died”.
The case seemed conclusive. Yet Butler was adamant: he had not met Foy.
“You do see an assumption being made that a DNA profile is evidence of contact – case closed – whereas it is actually a lot more complicated than that,” says Ruth Morgan, the director of the Centre for the Forensic Sciences at University College London. “We are only beginning to realise quite how complex it is.”
Since DNA was first used in a police investigation 31 years ago, to solve the murder of Dawn Ashworth, a 15-year-old schoolgirl who was raped and strangled in Leicestershire, the technique has attained an aura of being bulletproof. Certainly, in some cases, evidence of a DNA match to a suspect can be powerful. “There will be times when you get a really clear [DNA] profile, and it is very clear how that material got to the crime scene. And it is [also] very clear that it was during the course of an illegal activity,” says Morgan. “The classic [example] would be semen on the clothing of someone who is underage.”
But Butler’s case is…