— -- It's been more than 21 years since O.J. Simpson's ex-wife, Nicole Brown Simpson, and her friend, Ronald Goldman, were found stabbed to death in front of her Los Angeles home on June 12, 1994.
To date, the murder weapon has never been found. But the Los Angeles Police Department said today that it's conducting tests on a knife purportedly found at Simpson's old property -- the Rockingham estate in Brentwood -- after they recovered it from a retired LAPD officer within the past month.
Based on a preliminary assessment, an official briefed on the investigation does not believe the knife is the murder weapon. Tests were still being conducted.
Here's a look at what we've learned about the missing murder weapon over the years.
Cops Hunt for the Weapon in Chicago
In 1994, video from ABC station WLS in Chicago captures L.A. detectives wrapping up their Chicago-based search for the weapon near the hotel Simpson stayed the morning after the murders.
"Three days of searching in an adjacent vacant lot failed to turn up a murder weapon," WLS reported at the time, adding that sources said L.A. police were "turning to high-tech satellite technology to continue to search."
"Those sources say the satellite is being positioned to focus on Chicago and L.A. and can detect metal objects on the ground from high above in space," WLS reported.
An L.A. detective said at the time he thought the weapon was in Chicago, but wasn't optimistic about finding it.
Chances are "pretty slim," the detective said 10 days after the murder, adding: "it's never over until it's over."
Reports at the time said sources within the coroner’s office described the weapon as a large, serrated knife.
"The weapon was without question a large, serrated knife," WLS reported at the time, and "that weapon is believed to have been a 15-inch knife."
OJ Simpson's 15-inch Knife
The month before Brown Simpson and Goldman's deaths, O.J. Simpson bought a 15-inch stiletto knife with a stag handle from Ross Cutlery in downtown LA, near where he was filming a movie, the Los Angeles Times reported two weeks after the killings.
Prosecutors thought it could be the murder weapon and asked a coroner to compare that type of blade with the victims' wounds, the LA Times said today.
The defense then produced the knife in the preliminary hearing and forensic tests revealed that "the knife was in pristine condition, with no scratches or bloodstains to suggest that it had been used in a vicious double homicide," the LA Times said today.
Prosecutors in the criminal trial never introduced it as evidence, the LA Times said.
The Case Goes to Trial With No Weapon
After Simpson was charged with the murders, the case went to trial without a weapon.
Brown Simpson was likely stabbed four times in the neck before the final attack to her throat, medical examiner Dr. Lakshmanan Sathyavagiswaran testified at Simpson's trial in June 1995, the Chicago Tribune reported.
Sathyavagiswaran said that "all the slash wounds to her body could have been left by one single-edged knife about 6 inches long," the Tribune reported. "A double-edged knife could also have left some of the smaller wounds, but those cuts could also be attributed to the tapered tip of a single-edged knife," the Tribune said.
A single-knife theory was "central to the prosecution's contention that one assailant committed both murders in a short period of time," the Tribune reported.
On Oct. 3, 1995, at the end of a televised trial that captivated the nation, the former football star was acquitted of all criminal charges. Simpson was found liable for the deaths in a civil case in 1997.
The 'Kardashian Cover-Up' Theory
In 1996, ABC News' Barbara Walters asked Simpson's lawyer and longtime friend Robert Kardashian to address the famous Louis Vuitton garment bag that he was seen carrying from Simpson's house after the murders -- a bag that many speculated could have contained the murder weapon.
"I went to O.J.'s house just to support him," Kardashian told Walters. "I didn't know he was in Chicago. I just was standing there when he pulled up and walked right in."
Simpson's assistant then came out with the bag.
"I was raised to be a gentleman, I walked over and I said 'let me hold that for you,'" Kardashian said. "And then I walked up to the policeman standing guarding Rockingham and I said, 'I have Mr. Simpson's luggage.' And he said 'You can't come in.' I said, 'But it's his luggage will you take it?' And he said, 'No, you can't come in.'"
So Kardashian took the bag home. He said he then forgot that he had it and the bag "remained in the trunk of my car all night."
He told Walters he never opened it.
"It meant nothing at that point," Kardashian said. "Nothing about the bag came out until after he was arrested. And the bag was sitting, lying open in Mr. Simpson's bedroom in my home. The police were here, searched his room and the bag was lying open, just the way it was when he left."
Kardashian, who died in 2003, said he never saw or carried anything that looked suspicious.
"The police could have taken it [the bag] at any time. They never sought to do so," he said. "In fact when we turned it into the court 9 months later they still never did any tests to see if there was blood. I don't believe they really wanted to know the answer. I think it was better to leave speculation. And to let the public think there was something sinister about these bags."
Knife Said to be Found at Rockingham Handed Over to LAPD in Past Month
Today the Los Angeles Police Department said it's conducting tests on a knife purportedly found at Rockingham after they recently recovered it from a retired LAPD officer.
The LAPD is not releasing details on the description of the knife, Capt. Andrew Neiman of the LAPD said, but the lawyer for the retired cop said it was a 5-inch buck knife with a brown handle.
The knife was submitted to the lab to be studied for forensics including DNA and hair samples, Neiman said, adding that it’s unknown how long the investigation will take.
"The story, as it's told to us by the [retired officer] that we received it from," Neiman said, was that the officer "was working in the area of the Rockingham estate and he claimed that an individual" who said he was a construction worker gave him that knife.
The lawyer for the retired officer said a construction worker gave the knife to him in 2002 or 2003 while he was working security near Simpson's Rockingham estate, which was demolished in 1998. The officer, who had already retired at the time, called the LAPD, according to his lawyer, but was told the case was over. So he kept the knife, his lawyer said.
The LAPD did not have any immediate comment about the lawyer's claims.
Neiman said the double-murder case is open but, for Simpson, who was acquitted in the killings, “double jeopardy would be in place," Neiman said.
Neiman said the story told to police could be accurate, or the "whole story is possibly bogus from the get-go.”
Attorney Carl Douglas, who was a member of Simpson's legal team in 1995, told ABC News today that “the story is ridiculous.” Douglas is not currently Simpson’s lawyer.
There have been previous reports of a knife found near Simpson's old home.
The Associated Press reported in May 1998 that a construction crew found a folding-blade knife in Simpson's neighborhood but police said at the time that the knife couldn't be linked to the murders.
Police Department spokesman Lt. Anthony Alba said at the time that several knives had been found in Simpson's former neighborhood over the past few years but officials did not find any blood or other evidence linking to any crime.
ABC News reached out to Simpson’s attorney, Malcolm LaVergne, for comment.