A documentary film crew (History or Discovery Channel; I don’t remember which) did a meticulously accurate reenactment of the Kennedy assassination using crash-test dummies.
The fatal wound corresponded exactly to a shot fired from behind and to the left, in precisely the same trajectory as a round from the fifth floor of the School Book Depository would have followed.
Any shot fired from the grassy knoll (or from anywhere between Kennedy’s twelve and three o’clock) would almost certainly have struck Jackie too, in addition to spraying brain matter back and to the left.
It’s been pointed out time and time again that Oswald’s move was unscheduled and unannounced, and that Ruby was nearby only because he had just wired some money from a Western Union office up the street. He noticed the vehicles pulling into the underground garage and went to see what all the commotion was about. He just happened to be in the right place at the right time, and he often carried a pistol because he frequently dealt with large sums of money.