Shakur's mother, who died in May 2016 at age 69, spent her last years living with the horrific knowledge that her son's murder went unpunished

More than 20 years after being gunned down on the streets of Las Vegas, the murder of Tupac Shakur remains unsolved, denying his loved ones a sense of justice. His mother, Afeni Shakur Davis, who died in May 2016 at age 69, spent her last years living with the horrific knowledge that her son’s death went unpunished.

“She was, as you might expect of a mother whose child has been killed, very, very angry,” Donald David, who worked with Afeni to manage her son’s estate, tells PEOPLE. “She was angry because she felt that nobody was trying to find out what had really happened. She was angry that it happened, but she was more angry, I think, that it was trivialized.”

But according to an LAPD source familiar with the investigation, finding someone culpable in a murder is more difficult than grieving families believe. “They don’t understand the laws and how things work. It is not what you know, it is what you can prove.”

Las Vegas Police Department detective Dan Long, who is also involved with the case, agrees. “If you let everybody out there know what your investigation is and where you are going, it’s a problem,” he tells PEOPLE. “There are times when you hold things back. I know it sounds awful, but you have to.”

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The mystery surrounding Shakur’s death has spawned a complex tangle of conspiracy theories, compounded by an influx of false information. “Everybody wants notoriety. Everybody wants to be heard. Everybody wants to put their spin on it and make themselves look important,” says the LAPD source. “They will say whatever to make money. You have so many jailhouse informants trying to get favors and get out of jail. They’ve muddied the waters.”

For Detective Long, taking tip calls, many of dubious veracity, is a regular part of his job. “I get them weekly, daily it depends on the time of year. Obviously in September I get hundreds.”

In the immediate aftermath of the murder, many in the media speculated that the crime was the result of the raging East Coast/West Coast hip hop feud, which pitted Shakur against his rap rival, and former friend, Christopher “Biggie Smalls” Wallace (a.k.a. The Notorious B.I.G.). But police soon established a theory that Shakur’s death was the result of an altercation with a member of the Compton Crips street gang, in opposition with Shakur’s alliance with the conflicting Mob Piru (which evolved out of the Blood gang).

According to David, Afeni never subscribed to the theory that Smalls had ordered her son’s death. “What we came to believe was that it was not a West Coast/East Coast issue, but we came to believe that it was more a question of the Crips versus the Bloods.”

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The Crip who sparred with Shakur, Orlando Anderson, became a major person of interest in the case, but an investigation of his home turned up nothing. He was killed a year after Shakur’s murder in an unrelated shootout.

Anderson’s death put an end to a promising lead, leaving his mother—and millions of fans—to spend years wondering who did it. “She never felt a sense of closure,” says David. “There’s still no official determination as to who killed Tupac or even an effort on anybody’s part to find out.”

Barring the revelation of important new evidence, the LAPD source is doubtful the case will ever be formally solved. “[Anderson] was a suspect, but it doesn’t mean you can prove it. The case could only be solved in a court of public opinion.”