The FBI has released some of its secret files on the late Senator Edward Kennedy, who died in August 2009.
Most of the documents relate to the numerous, and at times specific, death threats made against Mr Kennedy.
The files contain little about the senator's controversial car accident on Chappaquiddick Island in 1969, in which a female passenger was killed.
The documents show that the FBI learned of the crash early on but kept the senator's identity secret.
Senator Kennedy was the last surviving brother of the famous political family.
President John F Kennedy was killed in Dallas, Texas, in November 1963, while Senator Robert F Kennedy was shot in a California hotel in June 1968 while campaigning for the presidency.
One letter in the FBI files warned that Ted Kennedy would be next, calling him "number three to be assassinated on Oct. 25, 1968. The Kennedy residence must be well protected on that date."
Many of the death threats against Ted Kennedy were made during his unsuccessful attempt to beat incumbent President Jimmy Carter for the Democratic nomination for president in 1980.
Others were traced to radical groups including the Ku Klux Klan.
One letter warns the senator to stay out of private planes and keep his bodyguards close, ominously concluding "your kids need a father".
The threats continued into 1985, the date of the latest files released by the FBI.
Ted Kennedy was the fourth-longest serving senator in US history, holding a seat for 47 years.
He died at the famed Kennedy compound in Hyannis Port, Massachusetts, after battling brain cancer. He was 77.
A highly effective lawmaker, his power and reputation earned him the nickname "the lion of the Senate".