In 1964 they could not call Allen Dulles and J. Edgar Hoover to testify.
Dulles and Hoover were too busy directing the Warren Commission coverup.
:-)
Okay, maybe not the best example since the Warren Commission was in charge of the coverup. Which, by they way, is important to remember when people rush to say the Secret Service, CIA, or FBI would NEVER try to take out a President.