L. Fletcher Prouty was the real Mr. X in Oliver Stone's film JFK. He made several allegations in the film - notably that Secret Service protection was lax for the presidential motorcade in Dallas, and that the Secret Service turned down an offer of help from military intelligence. He also made further allegations in his books about the assassination.
The Assassination Records Review Board (ARRB) interviewed Prouty in 1996. Guess what? He couldn't provide evidence for any of his allegations.
Here is the ARRB memo in its entirety.
There is a lot more about Prouty in my book, On The Trail of Delusion, Jim Garrison: The Great Accuser.