According to some researchers, conspiracy theorists consider four or five groups, alone or in combination, to be the primary suspects in the assassination of Kennedy: the CIA, the military-industrial complex, organized crime, the government of Cuba, and Cuban exiles. Other domestic individuals, groups, or organizations implicated in various conspiracy theories include Lyndon Johnson, George H. W. Bush, Sam Giancana, J. Edgar Hoover, Earl Warren, the Federal Bureau of Investigation, the United States Secret Service, the John Birch Society, and far-right wealthy Texans. Some other alleged foreign conspirators include Fidel Castro, the KGB and Nikita Krushchev, Aristotle Onassis, the government of South Vietnam, and international drug lords, including a French heroin syndicate.
Some conspiracy theorists have argued that Kennedy planned to end the involvement of the United States in Vietnam and was therefore targeted by those who had an interest in sustained military conflict, including the Pentagon and defense contractors.
Former Texas Senator Ralph Yarborough in 1991 stated: "Had Kennedy lived, I think we would have had no Vietnam War, with all of its traumatic and divisive influences in America. I think we would have escaped that."
According to author James Douglass, Kennedy was assassinated because he was turning away from the Cold War and seeking a negotiated peace with the Soviet Union. Douglass argued that this "was not the kind of leadership the CIA, the Joint Chiefs of Staff, and the military-industrial complex wanted in the White House."
Oliver Stone's 1991 movie JFK explored the possibility that Kennedy was killed by a conspiracy involving the military-industrial complex. L. Fletcher Prouty, Chief of Special Operations for the Joint Chiefs of Staff under President Kennedy, and the person who inspired the character "Mr. X" in Stone's movie, wrote that Kennedy's assassination was actually a coup d'état.
The House Select Committee on Assassinations reported that it investigated "alleged Secret Service complicity in the assassination" and concluded that the Secret Service was not involved. However, the HSCA declared that "the Secret Service was deficient in the performance of its duties." Among its findings, the HSCA noted: (1) that President Kennedy had not received adequate protection in Dallas, (2) that the Secret Service possessed information that was not properly analyzed, investigated, or used by the Secret Service in connection with the President's trip to Dallas, and (3) that the Secret Service agents in the motorcade were inadequately prepared to protect the President from a sniper. The HSCA specifically noted:
No actions were taken by the agent in the right front seat of the Presidential limousine [ Roy Kellerman ] to cover the President with his body, although it would have been consistent with Secret Service procedure for him to have done so. The primary function of the agent was to remain at all times in close proximity to the President in the event of such emergencies.
Some argue that the lack of Secret Service protection occurred because Kennedy himself had asked that the Secret Service make itself discreet during the Dallas visit. However, Vince Palamara, who interviewed several Secret Service agents assigned to the Kennedy detail, disputes this. Palamara reports that Secret Service driver Sam Kinney told him that requests—such as removing the bubble top from the limousine in Dallas, not having agents positioned beside the limousine's rear bumper, and reducing the number of Dallas police motorcycle outriders near the limousine's rear bumper—were not made by Kennedy.
In The Echo from Dealey Plaza, Abraham Bolden—the first African American on the White House Secret Service detail—claimed to have overheard agents say that they would not protect Kennedy from would-be assassins:
[President Kennedy] alienated Southerners and conservatives around the country, most of whom were already suspicious of him. In this, the Secret Service reflected the more backward elements of America. Many of the agents with whom I worked were products of the South.... I heard some members of the White House detail say that if shots were fired at the president, they'd take no action to protect him. A few agents vowed that they would quit the Secret Service rather than give up their lives for Kennedy.
Questions regarding the forthrightness of the Secret Service increased in the 1990s when the Assassination Records Review Board—which was created when Congress passed the JFK Records Act—requested access to Secret Service records. The Review Board was told by the Secret Service that in January 1995, in violation of the JFK Records Act, the Secret Service destroyed protective survey reports that covered JFK's trips from September 24 through November 8, 1963.