On 22 November 1963, President John. F. Kennedy was assassinated in Dallas, Lee Harvey Oswald was caught and presumed to be the killer, but a while later was shot dead by Jack Ruby and taken to Parkland Hospital, where he died. The very same hospital where the president died.
Later, the Warren Commission concluded that Oswald was the lone shooter, but many have refuted that, particularly the theory of the shooter on the grassy knoll, to the front-right of the president as the motorcade was passing by.
This date is considered by many as the birth of the conspiracy theory as we know it today and thousands of different viewpoints have emerged over the decades. Some just plain wacky and others, entirely believable.
Having just watched the film, Parkland and also having watched JFK years ago and more recently, LBJ, it sparked my interest again and I came across this documentary on ABC where long-time investigator, Josiah Thompson maintains that it was a professional hit both from the grassy know and the book depository. This infers that the grassy knoll shooter was the pro and that Oswald simply set up as a patsy - to take the blame.
Anyway, you catch it in full, here: