Los Angeles police are testing a knife found at the former home of OJ Simpson, whose acquittal at his 1995 murder trial captivated the US.
Someone claiming to be a construction worker gave an off-duty police officer the knife some years ago and he kept it, thinking the case was closed.
Police recently learned about the knife and recovered it, now testing it for DNA and hair samples.
Simpson had been accused of killing his ex-wife and her friend in June of 1994.
But a jury found him not guilty after a dramatic televised trial. He cannot be prosecuted again.
Police cast some doubt on the story at a news conference on Friday, saying it could be "bogus".
"If this story is accurate, you'd think anytime you come into contact with evidence that you should submit it to investigators," said Captain Andrew Neiman of the LAPD. "I don't know why that didn't happen."
Simpson later lost a wrongful death lawsuit brought by the victims' families. His property, where the knife was found, was razed in 1998.
The former professional US football player is currently in prison for an armed robbery conviction.
Simpson's ex-wife, Nicole Brown, and her friend Ronald Goldman, were found stabbed in the head and neck in the yard of Brown's home.
After authorities asked Simpson to surrender following the murders, he led them on a chase across Los Angeles motorways that ended up being broadcast on live television, driving a White Ford Bronco which became synonymous with the chase.
The case created a huge media frenzy, with television networks seeing high ratings from airing coverage of the case and countless magazine covers devoted to it.