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A tiny speck of blood only visible under a microscope cracked one of London's most controversial and long-running cases - the racist murder of teenager Stephen Lawrence.
Forensic scientist Dr Angela Gallop told ITV News London how she and her team found vital clues that no one had unearthed before.
The original police investigation failed to convict anyone and the campaign for justice by Stephen's parents led to a public inquiry.
Dr Gallop described how her team re-examined forensic evidence three years after the 18-year-old was fatally stabbed by a gang of racists in Eltham on April 22 1993.
"We had looked for it [blood] and not found it and other scientists before us had looked for it and not found it and we only found it by looking for something else first," Angela Gallop told ITV News London.
"And that's what we have found about doing re-investigations of cases, sometimes you have to go via an indirect route to find what you're looking for.
"We now understand much better about how to find things but we did not have the technology to analyse blood stains in that depth of detail in the decade before. "Even finding this blood stain took ages but you don't send a sample off and ten minutes the lab is ringing with the result!" she added.
The teenager's murder was a watershed moment in modern race relations after the Macpherson Report concluded police made mistakes and were guilty of "institutional racism".
Dr Gallop teamed up with the experienced detective put in charge of the case, Clive O'Driscoll.
Revisiting the scene of the murder with Clive, Dr Gallop said: "It’s the normality of it which makes it even more horrific in a way, isn’t it - a nice suburban street and that sort of awful thing that can happen."
Stephen’s parents, Neville and Doreen Lawrence, had doubts about the competence of the police investigation from the very beginning.
Their solicitor Imran Khan said: "I knew that police and the community and society suggest a stereotype of young black men. And he didn’t fit that at all. Stephen was a high achiever, he was going to be an architect. He hadn’t been in any sort of trouble."
An initial lack of forensic evidence dealt a massive blow to the family’s hopes of bringing a prosecution against the suspects in the original investigation.
And when the judge ruled that evidence from Stephen’s friend Duwayne Brooks identifying the suspects was inadmissible, the case collapsed.
But Dr Gallop and Clive realised something critical at the scene.
"You could begin to see that actually the attack would have taken place over a bit of a period of time and there would have been plenty of opportunity for particularly contact between clothing of the assailants and Stephen’s clothing," Dr Gallop explained.
Angela Gallop and her team explored the idea that there had been greater contact between the attackers and Stephen than first assumed.
She decided to focus on the possible transfer of textile fibres from clothing, saying: "We realised that Stephen’s polo shirt that he was wearing three or four layers down were shedding fibres. What it told us was that these red fibres might be worth looking for on the suspects’ clothing."
Among the clothes were items belonging to two of the original suspects, David Norris, and his friend, Gary Dobson.
But Dobson had already been acquitted of the murder in a trial which followed a private prosecution brought by Stephen’s family.
Forensic examiner April Robson said: "I started looking at Dobson's jacket for these red fibres, and I found eleven fibres that were very similar to Stephen’s polo shirt. They were then sent for additional analysis.
"So I then looked at the sweatshirt of David Norris and found one red fibre that was similar and again that was sent off for analysis. The analysis found that these fibres matched Stephen’s red polo shirt."
The team then found fibres from Stephen’s jacket on Gary Dobson’s clothing, and green fibres from Stephen’s corduroy trousers on the sweatshirt that David Norris had been wearing on the night of the attack.
But textile fibre evidence alone would not be enough to prove murder - so in a moment of inspiration they checked the packaging the clothing had been kept in, and found a flake of what after testing was found to be Stephen's blood.
Particle examiner Roger Robson said: "Finding that tiny fragment of blood… was huge. Almost something that you couldn’t imagine would happen other than in a drama. Blood had never been found before, so it was a great eureka moment."
The bag contained the jacket from suspect Gary Dobson and the team then found Stephen's blood on its collar.
A short piece of hair from Dobson's clothing was also sent to the United States to be analysed in a specialist laboratory - later being found to match Stephen's DNA.
Clive said: "That showed that they were there at the time Steven was attacked and Steven was murdered."
Despite his earlier acquittal, the new forensic evidence against Dobson was put before the Court of Appeal and the judges agreed it should be overturned.
After a six-week trial in January 2012, a jury found Dobson and Norris guilty of Stephen’s murder, receiving life sentences.
Clive said: "I can’t ever feel elation because I haven’t brought Stephen back, have I? I haven’t brought Stephen back - that’s what they would want. You know it was a feeling more of sadness for Baroness Lawrence and Neville Lawrence because of how much they’d suffered."
Dr Gallop added: "We are not the court, we don't judge anything we're just looking for evidence and then we present it for the court to decide what it means against the background of all the other evidence. "I think we knew we had a pretty strong case we had five different types of textile fibre, we had this tiny blood stain and we had some hair evidence as well. So quite a lot in the end."
You can read more about the work of Dr Angela Gallop in her book titled When the Dogs Don't Bark: A Forensic Scientist’s Search for the Truth
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