Using the word "contribute" is a good choice as it takes us away from conspiracy theories into reports of what the CIA, FBI and organized crime (usually referred to as the Mafia) are reported to have done in relation to events leading up to, during and after the assassination of John F. Kennedy.
Reports from at least one former CIA agent claim a connection between CIA orchestrated assassination attempts on Cuba's Fidel Castro (deceased November 25, 2016) and Kennedy's assassination. Reports connect organized crime to the CIA as the instruments of the assassination attempts. Reports connect the CIA to the FBI through the White House, where a White House liaison connected the activities of the CIA, mandated to operate outside the borders of the U.S., with the activities of the FBI, mandated to operate within the borders of the U.S.
Howard Hunt of the CIA and notorious for his role in the Nixon era Watergate break-ins, which resulted in Richard Nixon's resignation, reported that he and other CIA operatives were involved in an attempted Castro assassination stating that, after that and the Bay of Pigs, CIA agents were regrouped to form a within-borders domestic unit operating out of the White House. Richard Ober was reported as the White House liaison to the CIA and to the FBI, bringing extra-domestic and domestic law enforcement together. Hunt said to the New York Times in 1974, "Many men connected with that [Cuban] failure were shunted into the new domestic unit" in which his position was as Chief of Covert Action for the Domestic Operations Division of the CIA. It has been reported that on his deathbed Hunt confessed to his son that the CIA played an active role in Kennedy's assassination.
Mafia hitman James Files confessed in a documentary to firing the shot, from behind the grassy knoll, that killed Kennedy in Dallas. Files worked for organized crime leader Sam Giancana through Giancana's subordinate Charles Nicoletti. Files' confession is supported by a report made by retired FBI Special Agent Zack Shelton to documentary investigator Joe West about Files' involvement in the assassination.
The National Archives’ Special Access Branch is currently completing preparation of secret files on Kennedy's assassination scheduled to be released to the public in 2017. More than 3,000 of the 40,000 files due for release contain information, some of which was heavily redacted (blacked out and made illegible), exposing CIA operations during the Kennedy era and relevant to the CIA's role in Kennedy's assassination. Martha Murphy, director of the National Archives’ Special Access Branch, expects the CIA and other national security organizations will request presidential privilege to override the mandated October 2017 release of these files.
Lee Harvey Oswald and Jack Ruby
Warren Commission and U.S. House of Representatives Select Committee on Assassinations