Lots of theories, lots of conjecture, lots of documents, lots of crap fed to the Warren Commission, which wasn't convened to do much more than make sure that Oswald was the lone shooter.

LBJ had a special interest in just that, given that the 1964 presidential election was right around the corner. He wanted it wrapped up beforehand, tied up in a nice little bow so he could be seen as having "solved" the assassination.
I'm inclined to believe that the mob took care of JFK. Yeah, I know, it's more conjecture, but that one rings true to me especially given the circumstances (JFK's daddy, who was definitely associated with the mob) RFK's relentless attacks on the mob at that time (but nowhere near as successful as during Guiliani's tenure as DA of the Southern District of NYC, which came much later).
Michael Franzese talks pretty convincingly about all this but yeah, it's more conjecture.
I kinda doubt we'll ever know the real truth.