Instead of celebration, what would have been Tupac's 40th birthday has been marred by controversy as new allegations emerge about the November 1994 Quad Studios robbery and shooting of the rapper.

While fans celebrated 'Pac's life and legacy on Thursday (June 16), remembering the fallen icon with two trending topics on Twitter (#tupac and THUG LIFE), many of them were also debating the contents of a letter written by Dexter Isaac and posted to AllHipHop.com on Wednesday. In a note to the site, Isaac claimed to be the triggerman in the now-infamous robbery that left Shakur shot five times, two years before he was killed in a still-unsolved murder.

Moreover, Isaac (who is serving a life sentence for murder, robbery, fraud and witness intimidation), now charges that it was 'Pac's former associate, Jimmy "Henchman" Rosemond, who set the whole thing up, paying him $2,500 to do the job.

Rosemond's lawyer, Jeffrey Lichtman, fired back, refuting Isaac's story. "It's a flat-out lie," he told the New York Daily News. "Dexter Isaac is not claiming this 17 years later to clear his conscience. He's doing it because he's told anybody who will listen he doesn't want to die in prison. He has kids and wants to work off his sentence. He can't be trusted."

"I want to apologize to his family [Tupac Shakur] and for the mistake I did for that sucker [Jimmy Henchman]," Isaac told AllHipHop.com from prison. "I am trying to clean it up to give [Tupac's and Biggie's] mothers some closure."

Henchman and his defense team, however, are not buying it. On May 17, news broke that federal authorities issued an arrest warrant for Jimmy Henchman in connection with a drug case. Rosemond, who as head of Czar Entertainment has had ties to numerous rappers, believes that the government is using incarcerated informants to build a case against him. He attacked Isaac's credibility in a statement first issued to XXLMag.com. "If the government is relying on informants like Winston 'Winnie' Harris, a convicted drug dealer and Jamaican deportee, who came to me and motioned via hand signal that he was forced to wear a wire and begged me to skip town, or Dexter Isaac, who is serving life in prison plus 30 years, then I'm sure I will not be offered a fair trial."

The late Tupac's onetime protégé and a member of the rap group the Outlawz, E.D.I. Mean, told MTV News he wasn't surprised by Isaac's confession and pointed out that 'Pac himself also fingered Rosemond in his fiery 1996 song "Against All Odds." "I just feel like it's verification for what 'Pac said, because a lot of people felt when 'Pac was saying what he was saying on Makaveli that he was out of line for saying that."

In the lyric E.D. referred to, 'Pac implies that Henchman set him up in '94: "And did I mention, promise to pay back Jimmy Henchman in due time?/ I know you bitch n---as is listenin', the world is mine/ Set me up, wet me up, n---as stuck me up/ Heard the guns bust, but you tricks never shut me up."

Check back with MTV News for updates as this story develops.