Imprisoned one-time mob associate James Files claims in the new documentary I Killed JFK by Barry Katz that "he worked along with major mob figures and fired the shot that killed Kennedy from the grassy knoll at Dealey Plaza in Dallas" as reported by Jim Meyers for Newsmax:
Files, who changed his name from Jimmy Sutton, was born in Alabama in 1942, and after a stint in the Army became associated with both the Chicago mob and the CIA. He worked under mobster Charles "Chuckie" Nicoletti, an underling of mob boss Sam Giancana. Giancana and other mobsters had been angered by the president’s brother, Robert, who as U.S. attorney general was targeting organized crime in a major prosecution effort. * * * The documentary chronicles how Files, along with Nicoletti and mobster Johnny Roselli, came to be in Dallas on the day Kennedy was assassinated.
Retired FBI Special Agent Zack Shelton appears in the documentary, and "is one of two former FBI agents who believe Files' story is credible":
He said: "I have tried to verify Files' story. A lot of that story I have been able to verify," including that a shot was fired from the grassy knoll "exactly where Files said he was standing."
Historians increasingly have embraced the theory that the CIA and its Mafia tools were responsible for the 1963 assassination of President Kennedy, and among those is former FBI special agent William Turner who has dedicated his life to investigating the case as reported by KMAX: "'I believe the mafia was allied with the CIA in assassinating Kennedy, and I think we've proven that and should go ahead with closure -- designating it a conspiracy and not a single man's work,' said Turner."
Further reading that may be of interest: