Blood stained knife claimed to have been found at OJ's estate...

A retired police officer claims to be in possession of a blood stained knife buried at O.J. Simpson's old Rockingham Estate. TMZ reports below...

A construction worker found a knife buried on the perimeter of the former O.J. Simpson estate ... and it's currently being tested by the LAPD in a top secret investigation ... law enforcement sources tell TMZ.

The story is incredible. We're told a construction worker found the knife years ago -- we have heard several different stories, ranging from "several years ago" to 1998, when the house was demolished. The weapon is a folding buck knife.

Our law enforcement sources say the construction worker took the knife to the street, where he saw an LAPD cop. He told the officer where he found the knife and the cop took it.

Turns out the cop -- who worked in the traffic division -- was off duty at the time, working security for a movie shoot at a house across the street on Rockingham. Our sources say the officer took the knife home and kept it ... kept it for years.

In late January of this year, after the cop retired from the LAPD, he contacted a friend who worked in LAPD's Robbery Homicide Division (RHD). The cop told the friend about the knife and said he was getting it framed to put on his wall. He wanted his friend to get the DR (Departmental Record) number for the Nicole Brown Simpson/Ronald Goldman murder case, which he planned on engraving in the frame.

We're told the friend was indignant, and told his superiors. The brass was outraged and demanded that the retired cop turn the knife over, which he did.

Our sources say the knife is currently being tested for hair and fingerprints. It will be moved to the Serology Unit next week, where it will be tested for DNA and other biological evidence.

One source familiar with the investigation tells us, cops who eyeballed the knife think it could have blood residue on it, but it's hard to know without testing because it's extremely rusted and stained. The investigation is top secret. It's been logged into the LAPD's computer system outside the official case file to maintain security. Our sources say, since O.J. was found not guilty, it's still an open case. That means cops can continue investigating, but O.J. cannot be prosecuted again -- double jeopardy.