Philip Shenon explains on Fresh Air today how the FBI knew that Kennedy’s assassin Lee Harvey Oswald was a threat well before the shooting in Dallas:
It turned out that Lee Harvey Oswald had been under surveillance by the FBI for months before the assassination and the question becomes: Didn’t the FBI have information to suggest what a threat Lee Harvey Oswald might be? And didn’t it have an obligation to warn the Secret Service in advance of President Kennedy’s arrival in Dallas? … The decision seems to have been made by Hoover very early on to portray Oswald, whatever the evidence, as a lone wolf whose plot to kill the president could never had been detected by the FBI in advance — there was no conspiracy that the FBI could’ve stopped and saved the president. … The extent of the knowledge that the FBI had of Oswald before the assassination seems to be something that the FBI wanted to hide from the Warren commission.
Hear the full interview, read more highlights or read an excerpt from the book (about the botched autopsy) here