A key piece of evidence in the Stephen Lawrence murder investigation lay hidden in police files for 21 years because of a clerical error.
The black leather bag strap was recovered from the scene of the stabbing and is now being used in an appeal to find a new witness.
Detectives want to trace a female whose DNA profile was found on the strap.
They believe she may be connected to the gang of racist youths who chased the 18-year-old student and his friend Duwayne Brooks before stabbing him to death in Eltham, southeast London, in 1993.
The strap was recovered 11 yards from where Stephen was killed - but for more than two decades, it was wrongly logged as being found 90 yards away and considered of no relevance until the error was spotted two years ago.
Senior investigating officer Chris Le Pere said the position of the strap had been wrongly recorded "as an innocent mistake" and was not linked to police corruption allegations that dogged the first failed murder investigation.
Mr Le Pere added: "So far we have been unable to identify the owner of the bag strap. While there is no evidence that a woman was at the scene of the attack, I need to establish who this bag belonged to, and why was it at the spot where Stephen was attacked. The strap was not left by either Stephen or Duwayne Brooks."
The female may have been a victim of a theft, or the bag or strap may have been given to someone unwittingly, said Mr Le Pere.
He said he believes the strap was left at the scene by the suspects, with one possible theory being explored that it was an "adapted offensive weapon", much like one found at the home of a suspect.
Stephen's father, Neville Lawrence, said: "One small bit of information can bring it all together. I still can't understand seeing somebody at a bus stop, not knowing that person and that person hasn't done you any harm, but you and your friends decide to pounce on that person and take his life. I find it very, very hard to understand.
"I wanted my son to be famous from the buildings that he made, not from being dead."
Scotland Yard announced in January it was hopeful the potential new evidence would lead to a breakthrough, but has waited until now to reveal what it found.
Police asked 110 people connected to the investigation to volunteer DNA swabs so they could be eliminated.
Fifty agreed to give samples, 46 refused, 12 didn't reply and the rest were eliminated for medical reasons.
Two men - Gary Dobson and David Norris - were convicted and jailed for Stephen's murder in 2012 after DNA, hair and fibre evidence linked them to the killing.
Police believe there were up to six white youths involved in the unprovoked attack and are determined to identify other suspects.
The Metropolitan Police was widely condemned for its original failed investigation, which led to a public inquiry that resulted in the force being branded "institutionally racist" by judge Sir William Macpherson.
The judge concluded that alleged police corruption had not hampered the investigation.
Detectives are also still trying to identify another potential witness, a man wearing a distinctive V-emblem top who was caught on CCTV in an off-licence before the murder and seen later near the scene at the time of the stabbing.
The new appeal for information was made on BBC's Crimewatch on Monday night.
:: Anyone with information is asked to call the Met Police on 101 or Crimestoppers anonymously on 0800 555 111.