The documents showed that on May 23, 1985, the U.S. Capitol Police passed onto the FBI a copy of a letter sent to the Secret Service, ostensibly by a Warren, Michigan, resident. The sender, whose name was redacted, declared: "Brass tacks, I'm gonna kill Kennedy and (President Ronald) Reagan, and I really mean it."
The Federal Bureau of Investigation considered the sender armed and dangerous, but an accompanying psychological analysis said she was "merely ventilating her frustrations and projecting her inadequacies."
The 1985 threat was among 2,352 pages of documents the FBI posted on its website regarding the late senator, who died last year at the age of 77 after fighting brain cancer. Most of the documents are about death threats and extortion attempts against the Massachusetts Democrat.
"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 wrote on its website.
The release of the documents had been highly anticipated by historians, scholars and others interested in the life and long public career of one of America's most prominent and powerful politicians.
Kennedy faced death threats when he ran for president in 1980 and before that in the years following the assassinations of his older brothers.
President John F. Kennedy was assassinated in Dallas on Nov. 22, 1963. U.S. Sen. Robert F. Kennedy was slain in Los Angeles on June 6, 1968. Their deaths cast a long shadow on Kennedy's life, and prompted fears he too would be targeted by an assassin's bullet.
Letter number two, which had originally been received by police in Canada, read: "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 Oct. 25, 1968. The Kennedy residence must be well protected on that date."
After his brothers' assassinations, Kennedy wrote in his memoir "True Compass" released last year, that he was easily startled at loud sounds, and would hit the deck whenever a car backfired.
The new files also show that the FBI was told almost immediately of Edward Kennedy's car crash on Chappaquidick Island off the coast of Massachusetts, but authorities kept his identity secret.
The advisory said that Kennedy — the vehicle's driver — was uninjured.
It also says, "Stated fact Senator Kennedy was driver is not being revealed to anyone."
He swam to safety, leaving Kopechne in the car. Kopechne, 28, a former worker with Robert Kennedy's campaign, 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 on July 18, 1969, were "inexcusable." He said he was afraid and "made terrible decisions" and had to live with the guilt for more than four decades.
Kennedy, who served in the Senate for nearly half a century, died last August.
Associated Press Writer Kevin Freking also contributed to this report.