Legendary rapper Tupac Shakur knew his assassin - but his murder remained a mystery because police didn't care to investigate, a new documentary claims.

Tupac was gunned down on September 7, 1996, while driving through Las Vegas. Numerous people - including fellow New York rap hero Biggie Smalls, aka the Notorious BIG - were implicated in the death.

But the identity of Tupac's killer has remained a mystery - because the cops didn't care to look any further, TMZ reported.

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A new documentary says that rapper Tupac Shakur (left and right) knew who shot him - and admitted so on his deathbed. He was shot down in September 1996

The shocking remarks are made by Tupac's friends, family and music business colleagues in the episode of Snapped: Notorious Set to air on Oxygen on September 10 - almost exactly 21 years since his death.

The two-hour documentary paints a vivid picture of the night of the legend's death, TMZ said.

That includes interviews with those who were at the rapper's bedside as he lay dying - and indicated who his killer was.

Tupac, who lost a finger in the shooting, is said in the documentary to have nodded after being asked if he knew who opened up on him with a .40-caliber Glock.

But his killer was never caught nor tried for the murder, because of police inaction, his associates say.

Chief among the suspects is Notorious BIG, a former friend of Tupac's who became involved in a high-profile feud after they fell out.

In 1995, Tupac claimed that Biggie knew of a planned robbery that had resulted in Tupac being shot and losing valuable jewelry.

Tupac then signed to the West Coast's Death Row records, which was run by feared Compton boss Suge Knight and was in direct competition with Bigge's own Bad Boy records, based in Manhattan.

And three months before his death, Tupac released the track 'Hit 'Em Up,' in which he claimed to have slept with Biggie's then-estranged wife.

One suspect in the shooting is Notorious BIG (left), another New York legend and former friend of Tupac's who got into a beef with him after Tupack claimed to have slept with his ex-wife

Another assassin may be Orlando Anderson, who was beaten up by Tupac, Death Row Records boss Suge Knight (left) and their entourage the night Tupac was killed

But Bigge, who was himself gunned down in a drive-by shooting the following year, denied shooting Tupac, and claimed he was recording tracks when the hit took place - though those claims have been questioned since.

Another possible suspect, identified in a 2002 LA Times investigation, was Orlando Anderson, a member of the Southside Crips gang.

It claimed that Shakur, Knight and several of their entourage had beaten up Anderson the same day as the shooting.

That attack was revenge for Anderson and other members of the Southside crips for robbing a Death Row Records employee earlier in the year.

Las Vegas cops discounted the beating from their investigation, failed to follow up on claims by one of Tupac's friends that he witnessed the shooting, and didn't chase up a witness who may have seen the culprit's car, the LA Times reported.

Anderson, who denied the claims, was killed in a gang shooting in 1998.

But of course, other rumors persisted - including Knight's claim that the hit may have been intended for him in a Death Row Records coup.

Tupac himself reportedly refused to tell the cop who found him after the shooting who his killer was, simply saying 'f**K you.'

His killer may never be known, but some believe the truth is known to police - they just hated the rapper so much they'll never reveal it.

In 2014, EDI Mean, who collaborated with the rapper said: 'I'm absolutely positive they know what happened. This is America. We found Bin Laden.'