WASHINGTON - For decades after gunmen shot down his brothers, Sen. Edward Kennedy lived under constant assassination threats of his own, sometimes chillingly specific, as he became a target for extremist rage, previously private FBI documents disclosed yesterday.

The 2,352 pages of documents also contain reports of a rumor that the Mafia wanted to embarrass Kennedy by setting up a sex sting.

Five years after President John F. Kennedy was killed and shortly after Sen. Robert F. Kennedy was shot, one letter warned that the third brother was next: "Ted Kennedy number three to be assassinated on Oct. 25, 1968. The Kennedy residence must be well protected on that date."

Nearly two decades later, in 1985, the threats continued, this time including the Republican president, as well as the liberal Democratic senator: "Brass tacks, I'm gonna kill Kennedy and (President Ronald) Reagan, and I really mean it."

There is no indication any attempts were carried out.

In 1977, the FBI even looked into allegations that Sirhan Sirhan - who assassinated Robert Kennedy - had tried to hire a fellow prisoner to kill Edward Kennedy. The prisoner, who was housed next to Sirhan for 18 months, told the FBI he was offered $1 million and a car.

President Kennedy was assassinated in Dallas on Nov. 22, 1963. Robert Kennedy was slain in Los Angeles on June 6, 1968. Their deaths cast a long shadow on the youngest brother's life, and prompted fears he, too, would be targeted by an assassin's bullet.

He died last year at 77 after fighting brain cancer. There is relatively little on a major controversy, Kennedy's car accident on Chappaquiddick Island that killed Mary Jo Kopechne, who had been a worker in Robert Kennedy's campaign.

The files show that the FBI was told almost immediately of the accident and Kennedy's involvement, but authorities kept his identity quiet at the start.

The Boston FBI office relayed word to Washington headquarters at 2:45 p.m. EDT on July 19, 1969, after being notified by Police Chief Dominic Arena in Edgartown, Mass. The advisory said that Kennedy - the vehicle's driver - was uninjured. It also said, "Stated fact Senator Kennedy was driver is not being revealed to anyone."

Kopechne drowned after Kennedy drove the car in which she was riding off a bridge into a pond. He swam to safety, leaving Kopechne in the car. Kopechne, 28, was found dead in the submerged car's back seat 10 hours later. Kennedy, then 37, pleaded guilty to leaving the scene of an accident and got a suspended sentence and probation.

In his memoir, "True Compass," Kennedy wrote that his actions on Chappaquiddick were inexcusable. He said he was afraid, "made terrible decisions" and had to live with the guilt the rest of his life.

According to a memo released with the FBI files yesterday, an informant told the FBI in 1965 that it was rumored, though never substantiated, that mafia bosses hoped to use associates of entertainer Frank Sinatra to ensnare Robert and Teddy Kennedy in compromising positions with women in retaliation for a crackdown on organized crime.

The agent who sent the memo to the office of FBI Director J. Edgar Hoover described the information as "nebulous and vague."

"This is allegedly to be accomplished by working with outfit associates of Frank Sinatra to arrange for their women to be placed in compromising situations in the presence of any or all of the two Kennedys and Peter Lawford," the Aug. 3, 1965, memo stated. At the time Lawford, an actor who was part of the Sinatra "Rat Pack," was married to Kennedy sister Patricia Kennedy.

The FBI files include a one-page document generated about the same time that talks about sex parties at the Hotel Carlyle in New York City. Among those mentioned as participants were the Kennedy brothers along with Lawford, Sinatra, actress Marilyn Monroe, who died in 1962, and entertainer Sammy Davis Jr.