After two brothers fell to assassins’ bullets, Senator Edward M. Kennedy received repeated threats that he, too, would meet the same fate, according to thousands of pages of documents released Monday by the Federal Bureau of Investigation.

“A warning to the Kennedys,” read a letter sent in 1968 to the police in Edmonton, Alberta, one of scores of death threats against Mr. Kennedy. “John Kennedy number one assassinated, Robert Kennedy number two assassinated, Ted Kennedy number three assassinated on a set date of Oct. 25, 1968.’’

The threats, some of which resulted in investigations and another that merely resulted in warnings to Mr. Kennedy’s office, are contained in more than 2,200 pages of documents released by the F.B.I. covering 1961 to 1985. Some of the threats were made by members of radical groups, including the Ku Klux Klan, “Minutemen” organizations, and the National Socialist White People’s Party.

There is very little in the files on the 1969 Chappaquiddick incident, in which a young woman drowned after Mr. Kennedy, Democrat of Massachusetts, drove off a bridge on Martha’s Vineyard. “The F.B.I. had no investigative role in this case, since there were no violations of federal criminal law involved,’’ the bureau wrote in a summary, explaining that most of the documents in its files were clips from newspapers and other public sources.

But the file does contain a teletype to the director of the F.B.I. on July 19, 1969, flashing the first news of the drowning, and initially misidentifying the woman, Mary Jo Kopechne, as “Mary Palporki.”

“On this date Dominic J. Arena, chief of police, Edgartown, Martha’s Vineyard, advised body of female found in overturned car in water,’’ it read. “Tentatively identified as above, who was former secretary to former Senator Robert F. Kennedy. Mr. Arena confidentially advised that driver of automobile was Senator Edward M. Kennedy who was uninjured, stated fact Senator Kennedy was driver is not being revealed to anyone.’’

In his memoir, “True Compass,’’ Mr. Kennedy recounted writing a letter to the Los Angeles district attorney asking him not to seek the death penalty for Sirhan Sirhan, the assassin of his brother, Robert F. Kennedy. One of the F.B.I. files recounts the unsubstantiated claims of a prisoner who served time with Mr. Sirhan, and who said that Mr. Sirhan had tried to hire him to kill Mr. Kennedy. “He advised during this time subject offered him one million dollars and a car in exchange for killing Senator Kennedy,’’ according to the file.

There are many uncorroborated rumors and threats in the files. One of the more tantalizing ones, according to a summary by the bureau, contained a report of a rumor from an informant suggesting that organized crime wanted to attack “the character of Edward and Robert Kennedy and their brother-in-law Peter Lawford by working through associates of Frank Sinatra to compromise them at a New York party.’’

“In the convoluted rumor, both Frank Sinatra and Marilyn Monroe were to be involved,’’ the bureau wrote. “The F.B.I. 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.’’

There were also intriguing glimpses of the relationship between the Kennedys and J. Edgar Hoover, the longtime F.B.I. director.

In October 1964, Mr. Kennedy, then recovering from a near-fatal airplane crash that year, wrote Mr. Hoover to ask if he would contribute an essay for a book Mr. Kennedy was compiling of personal recollections about his father, Joseph P. Kennedy. “It is my hope that you would like to put down your thoughts about significant events involving my father, especially anecdotal accounts that would contribute to a profile of a man who has great warmth and affection and, at the same time, commands strong loyalties from those who know and have worked with him,” he wrote.

Mr. Hoover agreed, but only after aides combed their files for anything on the elder Kennedy. A memo on their search revealed that Mr. Kennedy had been a “special service contact” of the Boston office of the F.B.I. for 11 years and was on a first-name basis with Mr. Hoover. “The F.B.I. has never conducted a complete investigation of Mr. Kennedy; however, a limited inquiry was made in May 1951, at the request of the Department,’’ they wrote. “This concerned an allegation that Mr. Kennedy, then owner of the Merchandise Mart in Chicago, had given $100,000 for the removal of Government offices from that building. No specific facts were developed and the inquiry was closed on 6-7-51.”

The memo also noted that in 1953 the State Department had sent information about Mr. Kennedy, but the rest of the entry was blanked out.

A few weeks after receiving Mr. Kennedy’s request, Mr. Hoover sent an essay that covered seven double-spaced pages. Mr. Hoover noted Mr. Kennedy’s service on the new Securities and Exchange Commission and as the leader of a group trying to reform the Merchant Marine. He then recalled Mr. Kennedy’s service as ambassador to England, where he controversially said that England was not prepared to fight Nazi Germany and, in March 1938, opposed the United States’s entry into the looming war in Europe.

“For this,” Mr. Hoover wrote, “Joe Kennedy was accused of being pro-Nazi and anti-Semitic!”

Mr. Hoover wrote in 1955 that Mr. Kennedy had written him saying that he understood Mr. Hoover was interested in running for president. “He urged me to run for this position either on a Republican or Democratic ticket, guaranteeing me the largest campaign contribution I would ever get from anyone, and his personal services as the hardest campaign worker in history.”

Mr. Hoover respectfully declined. “I explained to him that I have never had any political ambitious and that I have always felt what contribution I can make to my country can best be given in my present position as Director of the Federal Bureau of Investigation.”