According to former New Orleans D.A. Jim Garrison's book, On The Trail Of The Assassins, "approximately an hour before the arrival of Kennedy's motorcade, Jack Ruby...was observed alongside the grassy knoll, unloading a man carrying a rifle in a case" and "the statement of Julia Ann Mercer, the witness to that event, was altered by the F.B.I. to make it appear that she had been unable to identify Ruby as the man."
According to Who Was Jack Ruby? by Seth Kantor, at the very moment JFK was shot, Ruby was "five blocks away" on the second floor of the Dallas Morning News building, complaining about the anti-JFK advertisements which, the newpaper had published that morning. According to the same book:
"...Some 10 minutes after the shooting, advertising salesman Richard L. Saunders encountered Ruby, standing numbly amid the confusion, uncharacteristically subdued and pale. It was Ruby's quietness that startled Saunders. He had known Ruby several years and always had found him to be an excessive talker, but Ruby was no more than barely responding to comments everyone else seemed to be making about how terrible the situation was."
Kantor also noted that "An hour after the shooting of President Kennedy I encountered Jack Ruby at Parkland Hospital" but "the Warren Report declared that `Kantor probably did not see Ruby at Parkland Hospital'" around the time JFK's death was confirmed.