The science that helped convict Gary Dobson

The key piece of evidence that convicted Gary Dobson was a microscopic spot of blood soaked into the fabric of his distinctive jacket discovered during an exhaustive £3.8m scientific review of the case.

The jacket – found by police hanging up in the back of Dobson’s wardrobe 15 days after the attack - was one of 30 items of clothing that were seized from the homes of suspects or recovered from Stephen Lawrence.

For years, the items had yielded few clues as to who murdered the black teenager. It was only when a new police team was appointed in 2006 that scientists were encouraged to re-scrutinise every piece of evidence again.

By 2006 there were no new exhibits to test. But a team from LGC Forensics, Britain’s biggest private laboratory slowly re-examined all the evidence again, checking for blood, hairs and fibres. They found all three.

When the jacket was seized, it was stored in an exhibits bag. The scientists had already checked the bag to see if anything had fallen from the jacket, but Ed Jarman, a blood expert at LGC, went further.

He considered the idea that debris might have fallen from the jacket and got trapped in the seals and folds of the paper bag. Using adhesive tape, he checked again in April 2008 – and the results were startling. He found a tiny bead of blood, with several fibres running through it.

The blood spot was sent away for analysis and when it came back, it was found to match that of Stephen Lawrence and two fibres from his cardigan. The chance of the blood not coming from the victim was less than a billion to one.

This was the breakthrough, and as close as the undemonstrative Mr Jarman would concede to a ‘Eureka’ moment. It was, he conceded, “a very significant finding.”

The discovery gave fresh impetus to the search. If a bead of blood was found, surely it must be more likely that more could be found on the jacket? At the time of the murder, the science was not far enough advanced to check for tiny specks of blood, so the scientists did not bother with anything more than a search with the naked eye. But from around 2000, DNA science was far enough advanced to put bloodstains of less than one millimetre through full DNA analysis.

The re-examination was also triggered by failures by the state-run Forensic Science Service (FSS) over its handling of exhibits in the case of Damilola Taylor, another young black victim of a knifing who was killed in 2000.

Scientists at FSS had failed to spot a 9mm spot of blood on the trainer of one of the killers. It took years of delays and three trials at the cost of £16m before two brothers were brought to justice.

The revelations prompted a public apology and a national review of cases from 2000 to 2005 that might need to be looked at again in light of errors by the FSS, which is due to close next year after it was found to be losing £2m a month.

That review was in part responsible for the decision by Cressida Dick – now the acting deputy commissioner – and Gary Pugh, the director of forensic services at the Metropolitan police, to order a full forensic review.

As part of that process, Mr Jarman embarked upon a painstaking examination of the jacket using a low-powered microscope that magnified the surface of the jacket by forty times.

It took a day and revealed a spot of blood no bigger than 0.5mms at its widest part, soaked into the collar. Tests showed it to be that of Stephen Lawrence. Scientists believed that it could only have got there from direct contact or if it flew from the bloodied blade of the murder weapon.

Investigators believed that the bloodspot was an immense breakthrough for the inquiry team. But this was 15 years on from the murder and had not been spotted before.

The 1999 Macpherson inquiry report said the scientists involved in the case were careful and conscientious and avoided criticism in the report. But could the blood have got there by any other way? At one point, the jacket was taken out of its protective packaging and laid on the floor of a cell to photograph.

The packaging was not designed to last for so long and in some cases, the tape had lost its stickiness. Could the hairs or blood flakes have got in by accident when packages were stored together?

A review of the case revealed a number of problems. At one point in 1993, the exhibits were mislabelled. Some of the officers that handled exhibits later visited the Lawrences at their home, raising the prospect of contamination. Some of the clothes were not immediately put into bags. And one officer on the team changed details on the system after being told he was being taken off the case after he got into a fight while on holiday in Spain. But the conclusion was that none of them could lead to contamination.

“In terms of the evidence not found (before), we don’t see that as an error,” said Alan Tribe, the Met’s evidence recovery unit manager. “The fact that it was found in 2007/2008 was based on evolving technology.”

* Stephen Lawrence: How the case breakthrough came
* A shrunken family: The first journalist to interview the Lawrences recalls the scene
* The science that helped convict Gary Dobson