More than 2,000 pages of documents detail the threats that plagued Kennedy throughout his 47-year Senate career.
Shortly after the murders of his brothers, the late senator received threats against his own life.
Such threats continued until well after he abandoned his 1980 bid for the presidency.
The FBI stresses that it never investigated Kennedy, even after the infamous car accident at Chappaquiddick.
The FBI released 2,200 pages of documents Monday from its files on Edward Kennedy detailing death threats made against the late senator in the decades after the assassinations of his two brothers.
The murders of President John F. Kennedy in 1963 and Senator Bobby Kennedy nearly five years later gave ominous weight to the repeated threats against their younger brother.
The declassified documents also touch on Ted Kennedy's 1969 car crash that killed a young woman, rumors of a Mafia plot to compromise him that was said to involve Marilyn Monroe, and the FBI's relations with his father Joseph Kennedy.
But they reveal little that was not already known.not "At no point do these files suggest that the FBI investigated Senator Kennedy for a criminal violation or as a security threat," the FBI said. "The bulk of this material concerns FBI investigation of threats of violence and other extortion claims against Senator Kennedy and other public officials."
Although Kennedy made an unsuccessful run at the presidency in 1980, fears that he too would be killed were ever present, and may have prompted his decision not to run again in 1984.
"These threats originated from multiple sources, including individuals, anonymous persons, and members of radical groups such as the Ku Klux Klan, 'Minutemen' organizations, and the National Socialist White People?s Party," the FBI said.
The documents also refer to threats from individuals angered by Kennedy's stance on Northern Ireland, and allegations of an alleged Mafia plot to kill the three Kennedy brothers.
Among them were letters threatening Kennedy just months after his brother Bobby was shot to death while campaigning for the presidency in Los Angeles on June 5, 1968.
"To whom it may concern, a warning to the Kennedys. John Kennedy number one assassinated. Robert Kennedy number two assassinated. Ted Kennedy number three to be assassinated on a set date of October 25, 1968," one of the letters said. "The Kennedy residence must be well protected on that date."
They were received by police in Edmonton, Alberta; the Edmonton Journal; and the FBI office in Seattle.
The young senator came to the FBI's attention early in his career with the files showing his father, Joseph Kennedy, called then FBI director J. Edgar Hoover about a rumor that a columnist was about to claim Ted had associations with communists.
But the bureau appears to have followed at arms length the greatest scandal in Kennedy's life -- the accident at Chappaquiddick in which the senator drove his car into a tidal channel and then fled the scene, leaving a young woman, Mary Jo Kopechne, to her fate in the sunken vehicle.
The FBI file consisted only of local press clippings of the accident, and a report from the FBI's Boston bureau.
The report said Edgartown police chief Dominic Arena "confidentially advised that driver of the automobile was Senator Edward M. Kennedy who was uninjured. Stated fact Senator Kennedy was driver is not being revealed to anyone."
One of the files reports a rumor from an informant that the Mafia was plotting to use associates of singer Frank Sinatra to attack the character of the Kennedy brothers and their brother-in-law Peter Lawford.
"In the convoluted rumor, both Frank Sinatra and Marilyn Monroe were to be involved," the FBI said.
It said the bureau "did not consider the rumor solid, and no other mention of it appears in the file, suggesting that the informant did not supply any corroboration to the story."