Shakur, who was signed to Knight's Death Row Records, died in September 1996, after he was hit in a drive-by shooting, aged 25.
His shock passing fuelled hip-hop's East Coast/West Coast war, which escalated once more in Marc, 1997 with the killing of Notorious B.I.G., aka Biggie Smalls.
Both slayings remain unsolved, and for decades, Knight has been at the centre of conspiracy theories regarding his alleged involvement in the tragedies, although he has always denied the speculation and has never been charged.
Now, in a newly-leaked transcript of a 2004 interview between Sizemore and FBI agents, obtained by The Sun, the actor claims to have received information from a Los Angeles gang member about Knight's reported role in the killings, and offered to help authorities set up a sting.
According to the documents, the "Black Hawk Down" star recalled crossing paths with Knight at an Alcoholics Anonymous meeting, and then joining him in a nightclub in 2001.
It was during the night out that Sizemore was allegedly approached by a gang member known as 8-ball, who spilled the details on the rap icons' deaths, and claimed to have contact with someone who could offer the FBI more information on the shootings.
"Sizemore will wear a wire or provide any information to help solve the Biggie Smalls murder," read the paperwork, uncovered by movie producer Don Sikorski as part of his research for his 2018 film "City of Lies (2018)", which revolved around the musicians' unsolved deaths.
Sikorski hopes the documents will prompt a closer look at the open cases - and maybe even lead to an arrest.
"When I was researching this case I found it very disenchanting because at the end of the day someone is dead - which is Biggie - and these documents exist but no one has seemed to care enough to look into them or put them out there into the public," he said.
"But I'm just one person so by putting these documents out there - I want to encourage other people to really look into this stuff."
Sizemore has yet to comment on the leak of the FBI interview.
Knight is currently serving a 28-year sentence for a fatal 2015 hit-and-run.