As the 50th anniversary of the JFK assassination draws nearer, the debate over who actually pulled the trigger rages on. Did Lee Harvey Oswald, as the Warren Commission concluded, use an Italian bolt-action Mannlicher-Carcano rifle to fire three shots from the Texas School Book Depository building, hitting President John F. Kennedy once in the neck and once in the back of the head? Or were there larger forces at work?

The belief that there was conspiracy to assassinate the president has only become more widespread with the passage of time. As ABC News reports, the House Select Committee on Assassinations concluded in 1978 that Kennedy was “probably assassinated as a result of a conspiracy.” A 2003 ABC News poll showed that 70 percent of Americans believed the JFK assassination was “not the act of a lone killer,” with 7 percent believing that Oswald was not involved at all.

So what is the truth? What really happened on Nov. 22, 1963, at Dealey Plaza in Dallas? We’ll probably never know, but we can continue to speculate. Below is a list of the top five JFK assassination conspiracy theories that have been bandied about over the years.

1.) The CIA Killed JFK

The Central Intelligence Agency has always been shadowy, mysterious organization, which lends itself perfectly to a conspiracy theory involving the JFK assassination. According to the Mary Ferrell Foundation, Kennedy made an enemy out of the CIA after the disastrous Bay of Pigs invasion of Cuba in April 1961. As a result of that failed operation, CIA Director Allen Dulles (who would later serve on the Warren Commission) was forced to resign and many allegedly began to see JFK as a threat to CIA interests.

As the New York Post reports, author Patrick Nolan surmises that the CIA wanted “power, self-preservation and to stop the Kennedys’ plan to make peace with Cuba and the Soviets.” Nolan theorizes that a group of rogue CIA agents, including Richard Helms (who became CIA director a few years later), James Angleton, David Phillips and E. Howard Hunt (of Watergate infamy), used three shooters placed at different locations in Dealey Plaza -- the Book Depository building, the Grassy Knoll and the Dal-Tex building -- to pull off the assassination. In his book, “CIA Rogues and the Killing of the Kennedys,” Nolan uses physical, medical and film evidence, as well as eyewitness accounts, to reach his conclusion.

2.) The Mafia Killed JFK

The relationship between organized crime and the Kennedy family stretched back decades prior to the assassination, as President Kennedy’s father, Joseph P. Kennedy, allegedly engaged in bootlegging during the Prohibition era of the 1920s. As the Los Angeles Times points out, it’s also believed that Joseph Kennedy used his Mafia connections to help his son win the crucial state of Illinois during the 1960 presidential election against Richard Nixon. However, the mutually beneficial relationship between the Mafia and the Kennedy family would soon come to an end.

According to ABC News, one version of this theory posits that Mafia was angered with JFK after the botched Bay of Pigs invasion, as they had hoped to re-exert their presence in Cuba, which had been erased after Fidel Castro rose to power. Additionally, JFK’s brother, Attorney General Robert F. Kennedy, had become a crusader against the Mafia in his position as the nation’s top cop.

As the Post reports, Lamar Waldron, author of the book “The Hidden History of the JFK Assassination,” theorizes that the assassination was masterminded by New Orleans mob boss Carlos Marcello, with help from Santo Trafficante of Florida and Johnny Roselli of Chicago. According to the Waldron, Marcello used Italian hit men, smuggling them into the country from Canada, to commit the crime. Marcello reportedly bragged about pulling off the assassination to an inmate at a prison where he was serving time in 1985. “Yeah, I had the son of a bitch killed,” he allegedly said. “I’m glad I did. I’m sorry I couldn’t have done it myself.”

3.) Lyndon B. Johnson Killed JFK

If one wants a definitive answer as to who killed JFK, look no further than the man who benefited the most from his assassination. At least, that’s what author Roger Stone says in his new book, “The Man Who Killed Kennedy: The Case Against LBJ.” In an interview with Voices of Russia, Stone says that LBJ -- who was sworn in as president on a plane in Dallas just hours after the JFK assassination -- used longtime associate and hit man Malcolm Wallace to do the deed. As the Post points out, Stone alleges that Wallace’s fingerprints were found on the sixth floor of the Texas School Book Depository Building, where the Warren Commission says Oswald fired his fatal shots.

According to Stone, Johnson insisted on both the trip to Dallas and the route through Dealey Plaza, where none of the buildings in the area were sealed off. Stone thinks LBJ both reduced the number of police officers on motorcycles on either side of the president’s car and ordered off the Secret Service agents that would have been riding on the rear bumper of the car. In the interview with Voices of Russia, Stone believes that LBJ can be tied to as many as eight murders in Texas prior to the JFK assassination, in order to cover up his corruption, voter fraud and more.

4.) The Russians Killed JFK

To examine the theory that the Soviet Union killed President John F. Kennedy, one must look at the geopolitical situation in 1963. This was the height of the Cold War and the peak of anti-Communist sentiment in the United States. According to ABC News, conspiracy theorists frequently cite the notion that Soviet Premier Nikita Krushchev was incensed at having to back down to JFK during the Cuban Missile Crisis the year before, and had the president killed by the KGB in retaliation.

Another interesting connection is Oswald’s time spent in the Soviet Union. In October 1959, Oswald -- a former Marine -- defected to the Soviet Union, where he met his future wife, Marina. Marina’s uncle was a colonel in the MVD, which is the Russian Interior Ministry service.

According to Stratfor, there are numerous anomalies about Oswald’s time in Russia, including how the two were able to get permission to marry (a requirement for any Soviet citizen marrying a foreigner), as well as why Marina, an upper-middle class woman with an uncle in the government, would agree to marry an American defector with little prospects whom she had just met one month before.

In early 1962, Oswald, Marina and their daughter left the Soviet Union for the United States. As Stratfor points out, Marina Oswald was granted permission to leave the the country with an American defector within weeks of her request, an extremely rare, if not completely unheard of, turnaround. Why? Conspiracy theorists point to questions like these as evidence of Soviet involvement in the assassination of JFK.

5.) The Cubans Killed JFK

There are two separate sub-theories surrounding Cuba and the JFK assassination. The first, as ABC News points out, is that Castro had JFK assassinated in retaliation for the numerous attempts on his life during the Kennedy administration, courtesy of both the CIA and the Mafia. In 1968, Johnson told ABC News that “Kennedy was trying to get to Castro, but Castro got to him first.”

During an interview with Bill Moyers in 1977, Castro said the theory was “absolute insanity.”

The second, perhaps more plausible, theory involves a mixture of militant anti-Castro Cuban exiles in Miami, the Mafia and the CIA. As the Mary Ferrell Foundation points out, the anti-Castro Cubans were enraged by Kennedy’s failure to provide crucial air support during the Bay of Pigs invasion. This could have been seen as emblematic of Kennedy’s “soft” approach to communism, a charge that had already been leveled at him after he chose not to invade Cuba during the Cuban Missile Crisis. Did anti-Castro Cubans, working in conjunction with the CIA and the Mafia, assassinate JFK?

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