21 November 2011 Last updated at 12:26 ET
A cardigan worn by Stephen Lawrence was among items of his clothing held as evidence by policeThe trial of two men accused of killing Stephen Lawrence has heard there was no written procedure to ensure forensic evidence avoided contamination.
In defence questions, a police exhibits officer said seized clothing was bagged but these were not always sealed.
Gary Dobson, 36, and David Norris, 35, both from south London, deny murder.
Prosecutors at the Old Bailey say DNA evidence links them to a group of white youths that attacked the black teenager in Eltham, south London, in April 1993.
The 18-year-old A-Level student was forced to the ground at a bus stop and then stabbed twice, the prosecution says.
BBC home affairs correspondent Matt Prodger said the reliability of scientific evidence was at the heart of the trial.
The prosecution has said textile fibres, and blood and hair matching Mr Lawrence was on clothing seized from the defendants in 1993 and discovered as part of a cold case review in 2007.
But in opening statements, lawyers for Mr Dobson and Mr Norris contended that there had been contamination of evidence by the police.
Robert Crane, a detective constable who was an exhibits officer at Eltham police station in 1993, told the court that items of clothing were placed in brown paper bags, but not always sealed.
He told the court that at the time he "had a degree of forensic awareness" and understood the potential for cross-contamination.
But he said there was no written procedure for ensuring that evidence was not contaminated and those handling the clothes wore gloves, but not the white forensic suits used today.
The jury heard that exhibits were stored in a disused cell in the building, and there were no designated forensic retrieval or packaging areas.
In cross-examination by Timothy Roberts QC, who represents Mr Dobson, the court was told that exhibit bags could be left open while officers were waiting for a photographer.
Mr Crane said he was not aware of any special procedures to prevent fibres or fragments being passed on to an exhibit by a police officer when they opened and resealed packages.
Meanwhile, Det Con Steven Pye, who collected the teenager's personal belongings from the hospital after he died, told the court that the items were in plastic hospital sacks and he bagged them up in police paper sacks using rubber gloves.
The court heard that some of the paper bags were not sealed at the hospital because blood stains on the items were still wet.
Mr Pye said the bags of bloodstained clothes were not immediately sealed because if the paper sacks became wet they might collapse. He passed them on to a colleague to be dried.
Mr Roberts, in cross examination, asked: "Is the upshot of all of this, however it happened, that the clothing from Stephen Lawrence that was most heavily blood stained, and therefore might contaminate other things, remained in unsealed packages whilst you dealt with it?
In response, Mr Pye said: "The most heavily bloodstained would appear to have been placed in paper sacks and folded over, yes."
Gary Dobson (left) and David Norris both deny murdering Stephen LawrenceThe court heard that Mr Pye had a stack of paper bags with him at the hospital that could have picked up fibres or blood flakes on the outside.
"By touching all of the packages with the same pair of gloves and folding over the tops of all the brown paper bags you could have distributed blood and fibres from each of these items on to the outside of the packages," said Mr Roberts.
"The items were placed in bags with the same pair of gloves. My recollection is that the bags were sealed at some point in the night but I can't remember when," replied Mr Pye.
The court was shown a graphic with the clothing worn by Stephen on the night he died to help them follow how the exhibits were stored and handled.
Jury members heard that Stephen's blue cardigan, black jacket, green body warmer, red t-shirt, white vest and green cords were taken to a drying room at Southwark police station.
A groundsheet was placed in the drying space to catch any debris, along with the paper bags used to store the clothes.
After three days in the drying room, the clothes were placed in new paper bags, the tops of which were folded over but not sealed because there was a need to photograph them at Eltham police station.
Mr Crane was questioned about the photographs.
'Nervous' when questioned
During cross examination Mr Roberts said: "At the time you weren't thinking about the risk of possibly picking up fibres or flakes by way of contamination."
"No that was the norm," replied Mr Crane.
Earlier, Graham Cooke an officer who questioned Mr Dobson while on house-to-house inquiries said the defendant had seemed nervous.
The retired police officer told the court that the defendant said he was at home all night studying on the night the teenager was killed, adding that he did not know the victim.
"In my opinion he was nervous at the time," said Mr Cooke.
The jury was also shown police surveillance photographs of the defendants outside a house in Bournbrook Road in Eltham.
The trial has been adjourned until Tuesday.
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