Simpson's ex-wife, Nicole Brown Simpson, and friend Ronald Goldman were found stabbed to death at the home in 1994
Los Angeles Police Department say they have recovered a knife apparently found at OJ Simpson's former estate where his ex-wife and her friend were murdered.
LAPD Capt Andrew Neiman said a construction worker purportedly discovered the blade years after the killings.
The builder handed it to an off-duty or retired traffic officer, he added, but detectives only recently learned of its existence.
Capt Neiman said they were conducting body fluid, DNA and hair sample tests on the weapon.
But he added that whatever evidence is found, Simpson probably cannot be tried again for the murders under America's double jeopardy rules.
OJ Simpson's ex-wife, Nicole Brown Simpson, and friend Ronald Goldman were found stabbed to death at the home on 12 June 1994.
The former professional football star and actor was acquitted a year later of two counts of murder in what media dubbed the trial of the century.
Police have never found the murder weapon.
Capt Neiman said it is believed the builder found the blade at the estate in Los Angeles' affluent Brentwood area, possibly in around 1998 when the home was being demolished.
The worker gave the knife to a traffic officer who was either off-duty or retired at the time and providing security for a movie shoot nearby.
But instead of taking the potential item of evidence to the precinct, the former officer allegedly kept it as a souvenir.
Capt Neiman told reporters: "I was really surprised.
"I would think that any LAPD officer would know that any time you come into contact with evidence you should and shall submit that to investigators."
He stressed that the story had not been verified.
TMZ.com, which first reported the story, said the weapon was a folding buck knife and it was found buried at the perimeter of the estate.
According to the celebrity gossip website, the LAPD learned about the knife in January this year when the retired officer contacted a friend in the department's homicide division.
He asked for the OJ Simpson case file number because he wanted to frame the weapon, reports TMZ.
In 1997, Simpson was found liable for the deaths of his ex-wife and Goldman in a civil lawsuit.
He was ordered to pay the victims' families $33.5m (€30.5m) in damages.
The case has received renewed attention recently because of FX television series The People v. O.J. Simpson.
Simpson, 68, is currently serving a prison sentence in Nevada for the 2007 armed robbery and kidnap of a sports memorabilia dealer.