One of the letters included an August 15, 1970 newspaper clipping from the Boston Globe that was sent to the senator's Boston offices. The writer wrote on the newspaper clippings: "Killer of Mary Jo," and "Mary Jo will haunt you... you are living in fear."
On the side of the page, the person also wrote, "Why to you kill Mary Jo SOB." On a page from the paper with obituaries the writer of the threatening letter writes, "You A**hole."
The FBI laboratory concluded that some of the threats were from the same person. On Oct. 22, 1970 the FBI noted that the U.S. Attorney's office had decided not to prosecute the case because it was not "a definite threat to injure the addressee or any other person."
Another letter contained newspaper clippings questioning why Kennedy didn't go to jail.
"Killer of Mary Jo. Someone gun him down like 2 brother," the letter read. "Wanted dead or alive Ted Kennedy, murder of Mary Jo rewards 50,000 dollars."
Another threat targeted Kennedy's brother-in-law, Sargent Shriver, when he ran as the 1972 Democratic vice-presidential candidate. The newspaper clip had rants against the Kennedy family including a reference to the senator: "Ted Killer of Mary Jo." The picture of Sargent Shriver had a circle around his neck, and the words, "A bullet this neck."
In subsequent years, Kennedy continued to receive letters warning him not to run for president or chiding him for the Vietnam War.
A postcard to his office from Pittsburgh in June 1969 urged him to vote for the Anti-Ballistic Missile treaty, or else: "Remember what happened to Bobby and Jack!"
A letter to Kennedy from Michigan in June 1969 said: "You are one of the richest Senators in the Senate. When you are dead you can't take it with you!"
Many of the threats were bizarre. For example, in December 1969, a letter from New Paris, Ohio, warned that 67 people would come to Massachusetts and harm Robert Kennedy's widow, Ethel, if Ted did not come to New Paris at a certain time later that month and meet with residents at a local Dairy Queen.
The threats continued well past the 1970's. The senator's office received a letter in 1985: "Brass tacks, I'm gonna kill Kennedy and (President Ronald) Reagan, and I really mean it."
The FBI today released more than 2,000 pages of documents from Kennedy's files, dated from 1961 to 1985. Kennedy himself was never investigated by the FBI and the agency was not asked to provide support to the local police investigation into the Chappaquiddick incident.
Kennedy was the driver of the car that drove off a small bridge and killed Kopechne, then 28, on the night of July 18, 1969. Kennedy, who walked away from the scene and was initially not identified as the driver, maintained his innocence in the accident but the scandal plagued his political career and personal life.