U.S. Senator Ted Kennedy (D-MA) reacts after President Barack Obama signed H.R. 1388, the Edward M. Kennedy Serve America Act, at the SEED Public Charter School in Washington, April 21, 2009.

Late U.S. senator Edward Kennedy, whose two elder brothers were assassinated in the 1960s, lived with constant threats on his life, according to FBI files released on Monday.

"The bulk of this material concerns FBI investigation of threats of violence and other extortion claims against Senator Kennedy and other public officials," FBI said in the previously confidential files totaling more than 2,200 pages, which cover the FBI's relationship with the Massachusetts Democrat from 1961 to 1985.

"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 said.

Edward Kennedy had served as U.S. senator for the sate of Massachusetts from 1962 until he died of cancer in August 2009. His two brothers, President John F. Kennedy and Sen. Robert F. Kennedy, were assassinated in 1963 and 1968, respectively.

Among the newly released documents were letters threatening Kennedy just months after his brother was shot to death in 1968.

"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 a set date of October twenty-fifth nineteen sixty-eight," one of the letters said. "The Kennedy residence must be well protected on that date."