It is hard to believe that it has been 20 years since rapper Tupac "2Pac" Shakur's murder. Even harder to believe: 20 years later, the identity of Shakur's killer still remains a mystery. 

Shakur was gunned down in Las Vegas in 1996 at the age of 25 after leaving a boxing match between Bruce Seldon and Mike Tyson. He left behind an incredible legacy and an indelible mark on rap music, but many fans still obsess over the foggy details of the Shakur's death. 

The shooting occurred at 11:15 p.m. local time when a car carrying Shakur and Death Row Records CEO Suge Knight stopped at a red light at the intersection of East Flamingo Road and Koval Lane. Shakur was shot four times by a .40-caliber Glock, twice in the chest, once in the hand, and once in the thigh. He died from his injuries six days later in the University Medical Center of Southern Nevada.

tupac-2 Tupac Shakur, whose real name was Lesane Parish Crooks, was born in East Harlem, New York City in 1971. He died Sept. 13, 1996 when he was gunned down by an unknown assailant in Las Vegas. Photo: Reuters

One of the most prominent suspects in the shooting was Crips gang member Orlando Anderson. Anderson was involved in a gang-related scuffle after the Sheldon-Tyson fight during which Shakur and Knight allegedly assaulted Anderson. The fight led many to suspect that Anderson had motive to get revenge on Shakur. 

Tupac's former bodyguard, Frank Anderson, has been quoted as saying: "If you get into a scuffle and you get your butt whipped, what are you gonna do? You're going to tell your friends, 'Hey I just got into a fight and guess who it was with? Tupac.' And someone's going to say, 'You know what, let's go get some payback."

Anderson was never charged and always denied that he was involved in the shooting. He was killed in a gang shooting in 1998.

Others allege rapper Biggie Smalls, better known as The Notorious B.I.G., hired someone to kill Shakur. Shakur and Smalls were at the center of the contentious East Coast-West Coast hip-hop rivalry and some say the feud boiled over into violence. A 2002 Los Angeles Times report alleged that Small paid a million dollars for the hit. Many who buy into this theory further believe that Knight was behind Small's murder just six months later in March 1997 as an act of revenge. 

The again, some believe Knight was responsible for Shakur's death as well. A conspiracy theory pushed in the 2002 documentary "Biggie and Tupac" claims that Knight had Shakur killed and framed Anderson and the Crips gang for the crime. The possible motives for this conspiracy range from the possibility that Shakur planned on leaving Death Row Records to a plan to cash in on the publicity and notoriety Shakur would gain in death. 

Tupac Rap music star Tupac Shakur died September 13, six days after being shot four times while riding in a car in Las Vegas. Shakur had been in critical condition since he was shot and had a lung removed in one of two operations. Shakur is shown here at the MTV Music Video Awards in New York September 4. Photo: Reuters

Former LAPD detective Greg Kading, who led a special task force that investigated the murder, claimed that music mogul Sean “Puff Daddy” Combs paid Crips gang member Duane Keith “Keffe D” Davis a million dollars to kill Shakur after the altercation with Anderson, according to the 2015 documentary "Murder Rap." Other conspiracy theories pin the murder on the Jewish Defense League — FBI reports indicate that there was bad blood between Shakur and the pro-Israel group, which may have been trying to extort money from the rapper — and even the Illuminati

The most famous conspiracy theory of all is that Shakur was never killed and is still alive, living out his days abroad, possibly in Cuba.