The Warren Commission came to the conclusion that John F. Kennedy was assassinated by a lone gunman, Lee Harvey Oswald. This theory has been supported by several other investigators including Arlen Specter, Walter Cronkite, Dan Rather, Hugh Aynesworth, Gerald Posner, John McAdams and Kenneth A. Rahn.
David E. Scheim has published two books claiming that the Mafia were responsible for the assassination of Kennedy. He believes that it was organized by Carlos Marcello, Santos Trafficante and Jimmy Hoffa. This theory is based on the idea that the Mafia were angry with both John F. Kennedy and Robert Kennedy for their attempts to destroy the Mafia. Scheim's theory was supported by Trafficante's lawyer, Frank Ragano, who published the book Mob Lawyer, in 1994. The theory is also supported by the investigative journalist, Jack Anderson. G. Robert Blakey, chief counsel and staff director to the House Select Committee on Assassinations from 1977 to 1979, published The Plot to Kill the Presidentin 1981. In the book Blakey argues that Lee Harvey Oswald was involved but believes that there was at least one gunman firing from the Grassy Knoll. Blakey came to the conclusion that the Mafia boss, Carlos Marcello, organized the assassination.
James Angleton believed that Nikita Khrushchev sought revenge after he had been humiliated by Kennedy during the Cuban Missile Crisis. In his book, Khrushchev Killed Kennedy (1975), Michael Eddowes argued that Kennedy was killed by a Soviet agent impersonating Lee Harvey Oswald. In Legend: The Secret World of Lee Harvey Oswald (1978), Edward Jay Epstein argues that Oswald was a KGB agent.
Jim Garrison, the district attorney of New Orleans, believed that a group of right-wing activists, including Guy Bannister, David Ferrie, Carlos Bringuier and Clay Shaw were involved in a conspiracy with the Central Intelligence Agency (CIA) to kill Kennedy.
Stephen Riveleargued in the 1988 television documentary, The Men Who Killed Kennedy that the Kennedy's assassination had been organized by Antoine Guerini, the Corsican crime boss in Marseilles. He also claimed that Lucien Sarti had been one of the gunmen.
Madeleine Brown claims that she was Johnson's mistress. In her autobiography, Texas in the Morning (1997) Brown claims that the conspiracy to kill Kennedy involved Lyndon B. Johnson and several Texas oil men including Clint Murchison, Haroldson L. Hunt and J. Edgar Hoover. Joachim Joesten, an investigative journalist, believes that Johnson's secretary, Bobby Baker was involved in this plot. This theory was supported by Craig Zirbel in his book The Texas Connection: The Assassination of President John F. Kennedy (1991).
Gaeton Fonzi was a staff investigator for the House Select Committee on Assassinations. In his book, The Last Investigation, Fonzi takes the view that the assassination was organized by David Atlee Phillips, head of the CIA's Western Hemisphere Division.
David Atlee Phillips, head of the CIA's Western Hemisphere Division, told Kevin Walsh, a former investigator with the House Select Committee on Assassinations: that Kennedy had been "done in by a conspiracy, likely including rogue American intelligence people."
The journalist, Dorothy Kilgallen, believed that the assassination of Kennedy had involved Jack Ruby and the Mafia. She also suggested that J. D. Tippet and Bernard Weismann were involved in the conspiracy.
Anthony Summers is the author of The Kennedy Conspiracy. He believes that Kennedy was killed by a group of anti-Castro activists, funded by Mafia mobsters that had been ousted from Cuba. Summers believes that some members of the CIA took part in this conspiracy. Summers speculated that the following people were involved in this conspiracy: Johnny Roselli, Carlos Marcello, Santos Trafficante, Sam Giancana, David Ferrie, Gerry Patrick Hemming, Guy Bannister and E.Howard Hunt. Sylvia Meagher in her book, Accessories After the Fact, also supported the theory that Kennedy had been killed by Anti-Castro exiles.
Executive Action, was a CIA secret plan to remove unfriendly foreign leaders from power. In his bookThe Secret Team (1973) Leroy Fletcher Prouty claimed that elements of the CIA were worked on behalf of the interests of a "high cabal" of industrialists and bankers. He also claimed that the Executive Action unit could have been used to kill Kennedy. Prouty named CIA operative, Edward Lansdale, as the leader of the operation.
In his book, Best Evidence, David Lifton claims that members of the Secret Service agents were involved in the killing of Kennedy. This included providing the assassins with a good opportunity to kill Kennedy. Lifton was highly critical of the behaviour of William Greer, Roy Kellerman and Winston G. Lawson during the assassination. Lifton believes that after the assassination of Kennedy they hijacked the body in order to alter the corpse. In the book, Mortal Error, Bonar Menninger, claims that SS agent George Hickey killed Kennedy by accident.
J. Edgar Hoover was concerned that Kennedy would force him into retirement when he reached the age of 70. Mark North (Act of Treason) and George O'Toole (The Assassination Tapes) both believe that Hoover either knew of plans to kill Kennedy and did nothing to stop them, or he helped to organize the assassination. In his book, Deep Politics and the Death of JFK (1993) Peter Dale Scottprovides information that Hoover and the Federal Bureau of Investigation helped to cover-up the real identity of the people who assassinated John F. Kennedy.
Harry Dean was an undercover agent for the Federal Bureau of Investigation. In 1962 he infiltrated the John Birch Society. He later reported that the society hired two gunman, Eladio del Valle and Loran Hall, to kill President John F. Kennedy.