Detectives investigating the murder of teenager Stephen Lawrence more than 20 years ago have approached "a number of people" to give new DNA samples.
Scotland Yard said that "advances in forensic examination and DNA profiling" since the brutal stabbing of Stephen, 18, had led officers to focus on an item recovered on the night he died, April 22 1993.
But they added that the people asked to voluntarily give DNA samples "are not suspected of being involved in the murder".
A Metropolitan Police spokesman said: "Officers from the homicide and major crime command investigating the murder of Stephen Lawrence have contacted a number of people to ask them to provide a voluntary DNA sample to police.
"Due to advances in forensic examination and DNA profiling, police inquiries are focusing on the significance of a certain item recovered on the night of the murder.
"The individuals who police have approached for DNA samples are not suspected of being involved in the murder.
"We will not discuss what the item recovered on the night of the murder is, or who we have approached for a DNA sample."
Stephen, a would-be architect, was stabbed to death by a gang of white youths as he waited with a friend at a bus stop in Eltham, south-east London.
It was more than 18 years before two of his killers - Gary Dobson and David Norris - were brought to justice.
The pair were jailed for life in January 2012 for their role in the attack.
The 1998 Macpherson Inquiry into the police handling of the original murder inquiry found evidence of "institutional racism" within the Met.