Edward Kennedy died last year of brain cancer at 77, after a long political career at least partially redeemed the disgrace of fleeing his Chappaquiddick car accident, but the Massachussetts senator was subject of scores of death threats before and after the assassination of his brothers, documents released by the FBI revealed last night.

Five years after Jack was gunned down in Dallas and soon after Robert was shot in Los Angeles, one letter warned he was next. "Ted Kennedy number three to be assassinated on Oct. 25, 1968. The Kennedy residence must be well protected on that date."

In 1985, he and President Ronald Reagan were named in another threatening missive. "Brass tacks, I'm gonna kill Kennedy and Reagan, and I really mean it."

The FBI has released more than 2,200 pages of documents after multiple freedom of information requests, saying on its website: "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."

Some threats were passed on to law enforcement agencies and Kennedy himself but there is no indication any were anything other than threats.

The FBI did investigate at least one itself, concerning allegations that Robert's killer, Sirhan Sirhan, had attempted to hire a fellow prisoner to end Edward's life, too. The prisoner told the FBI he was offered $1m and a car but declined.

Kennedy revealed in his book True Compass that, after his brothers' violent deaths, he was startled by loud nouises and would dive to the ground when a car backfired.

The FBI documents reveal relatively little about the 1969 accident at Chappaquiddick Island, off the Massachussetts coast, in which a young woman, Mary Jo Kopechne, died. Kennedy was driving when the car left a bridge over a pond. He swam to safety, leaving his 28-year-old companion who was not found until 10 hours later.

The files show the FBI was told almost immediately of the accident and Kennedy's involvement but kept his identity quiet at the start. The Boston FBI office relayed word to Washington headquarters on 19 July, 1969, after being notified by Dominic Arena, the police chief in Edgartown, Massachusetts. The report said: "Stated fact Senator Kennedy was driver is not being revealed to anyone."

The FBI said last night however it had "no investigative role in this case, since there were no violations of federal criminal law involved. Although there was a mistaken contemporary report about FBI involvement in the case, the Bureau was not asked to provide support to the local police investigation."

Kennedy, then 37, pleaded guilty to leaving the scene of an accident and received a suspended sentence and probation. He later wrote that his actions were inexcusable, but he was afraid. He said he "made terrible decisions" and had to live with the guilt the rest of his life.