I have read several excellent books, websites and watched some great documentaries on the Kennedy assassination. I am quite sure that Oswald killed Kennedy and that he acted alone. I do not understand the emotional need that many have for believing in a massive conspiracy. Every major conspiracy theory has been dissected and convincingly refuted at John McAdams's site "The JFK/Kennedy Assassination Home Page." For those interested in the JFK conspiracy theories, this site is fascinating.
Lately we have seen a revival of a theory that first appeared in 1992 with the publication of a book called "Mortal Error." This book advances the absurd theory that the fatal head shot was fired accidentally by a Secret Service agent. The agent, one George W. Hickey Jr, sued the author and the publisher for libel and settled out of court for a huge sum. The book was legally libelous because Hickey was still alive when the book was first published. Now that Hickey is dead, the theory has been revived and is again being foisted on a gullible public.
This idiotic theory of the "accidental head shot" is nonsense, but there is no end to absurd assassination theories, and no limit to the gullible and credulous who will believe most anything, the Warren Report being the sole exception.
That Hickey could have fired an AR-15 into Kennedy's head with hundreds of spectators watching (and somehow not seeing) is specious enough. But neutron activation analysis of the lead fragments in Kennedy's brain and wounds showed all of them came from Oswald's rifle. There were no lead fragments from any other firearm.
UPDATE: This "Mortal Error" theory seems to be based on the belief that a second kind of gun (other than Oswald's) had to have fired the fatal head shot. The reason: the second bullet fired passed through both Kennedy and Connally without fragmenting, whereas the third bullet that passed through Kennedy's head badly fragmented, causing a massive exit wound. This theory ignores the fact that the second bullet was slowed by its passage through the soft tissue of Kennedy's throat (emerging just under his Adam's apple), and began tumbling end over end. The lost velocity, as well as the absence of anything hard (like bone) in the bullet's path, was enough to prevent fragmentation upon hitting Connally (the bullet did scrape a rib). The third bullet experienced no such impediment and hit Kennedy's head at full force, approximately 2100 feet per second. Therefore, the flight paths and velocities of the two bullets are not comparable, and the assumption that a second gun fired the third bullet both illogical and counter factual.
UPDATE: Per Bugliosi, the third (head shot) bullet did fragment, but two pieces of it were found on the front floor of the limousine (again, possible only if the head shot were from the rear). The FBI found the two pieces to be large enough for analysis, and they showed the rifling peculiar to Oswald's Carcano rifle, proving that the third shot, the head shot, was fired from Oswald's rifle and no other. Certainly not from an AR-15. "Mortal Error" is a glaring lie, and the authors should be ashamed for introducing yet another myth into the assassination.