The theory that a bloodstain on the jacket of a man accused of murdering Stephen Lawrence was caused by contamination is “practically impossible”, a court has heard.
Forensic expert Rosalyn Hammond gave evidence to a jury at the Old Bailey last week.
She dismissed the idea that the stain on defendant Gary Dobson’s jacket was caused by an old flake of blood being dissolved during saliva testing.
She said, “In my opinion this explanation is so unlikely as to be practically impossible.”
Dobson and David Norris deny taking part in the gang attack in which Stephen Lawrence was killed in Eltham, south east London, in April 1993.
The prosecution argues that tiny amounts of fibres, blood and hair found on clothes seized from their homes prove they were involved in the 18-year old’s murder.
The defence says the samples got on to the clothes through contamination during handling and storage.
The forensic evidence includes a tiny stain of Stephen’s blood found on Dobson’s collar.
Stephen’s clothes were stored in a disused police cell at Eltham police station.
His jacket was photographed on a groundsheet on the floor of the building on 26 April 1993.
Hammond discounted the possibility that debris had fallen onto the floor, got on to the packaging of suspect exhibits and later transferred onto the items themselves while at a police lab.
Stephen’s clothes were taken to the laboratory nine days before the Norris and Dobson exhibits were stored in the cell.
Hammond also dismissed the possibility that family liaison officers who had visited the Lawrence family and then searched Dobson and Norris’s houses had contaminated exhibits.
Dealing with DC Linda Holden, who searched Dobson’s house, Hammond said, “Considering the fibre evidence on LH5 (Dobson’s jacket), a total of at least 13 matching fibres, in my opinion it’s not possible that this could have transferred to and been recovered from LH5 by this route.”
Earlier forensic scientist Roy Green gave evidence to the jury. He said one of the scenarios he considered was that Norris had visited one of the killers and picked up fibres on his clothes.
A team of experts found six green fibres matching Stephen’s trousers and one matching his T-shirt linked to a sweatshirt seized from Norris’ house.
Stephen Batten QC, for Norris, asked Green, “What you postulated to yourself is that he might know one of the real assailants for example, and have gone round to their address?”
Green said, “That was one of the scenarios that I considered, yes.”
The fibres expert told the court that he had also looked at the possibility that the sweater was washed after the attack.
Batten told him, “You haven’t a clue whether it was washed or not,” and said he had only mentioned it because there were only a few fibres found.
Green said, “It was offered as one of the many scenarios which I had considered.”
The fibres were found distributed all over the sweatshirt, which Green said might be due to the garment being moved around after it was seized.
He added, “We do have to remember that what we see here is what remains after other fibres have fallen off and so it doesn’t necessarily give a completely true picture of what was originally there.”
The trial continues.