I am very much aware of many Secret Service "deficiencies," (to be kind) in the JFK assassination. By comparing the Zapruder film with the Orville Nix film, it is clear that when shots were first fired at the President, driver William Greer braked the limousine to almost a complete stop, and, only after the fatal head shot, do the brake lights go off and the car accelerates. All this in violation of SS regulations which state at the first sign of trouble, accelerate and drive out of danger. It is also clear that Greer turns his head and looks back at Kennedy at the time of the fatal head shot which is at odds with his Warren Commission testimony, in which he claimed he never looked back and did not even know the assassination had taken place until Roy Kellerman next to him said, "Let's get out of here, we're hit!"
However, the driver-shot-Kennedy theory is not true, primarily because it simply didn't happen. I have been aware of this issue for many years and have studied it at great length and detail. Think about it. If Greer had shot JFK, someone in the crowd, some less than then 10 feet away, would have said something. Other SS agents and others in the know would have whispered about this for years. Rumors would have floated around Washington and Nellie Connally, whom I knew personally, would have talked, at least as gossip. None of this happened.
In fact this issue never came up until about the mid-1980s when William Cooper latched onto a hypothesis of researcher Lars Hanson of California, who, upon viewing a bad 4th or 5th generation copy of the Zapruder film, speculated that the driver turned and shot Kennedy. Upon careful inspection of the film and further reflection, Hanson denounced his own theory but this did not stop Cooper from selling bad (some so bad there was no color) copies of the Zapruder film and continuing to claim the driver shot JFK even though Hanson and several other JFK researchers, myself included, warned him not to do so because the claim was false. Cooper would not listen and sucked in many other credulous researchers until he was killed after shooting at local police who came to serve him a warrant in 2001. In addition to his claim that Greer shot JFK, Cooper also claimed that all the nearest witnesses to the JFK head shot were dead within a few years of the assassination, a claim demonstrably false. I do not want to disparage the dead, but poor Cooper had some problems and he was certainly wrong about Greer.
So what exactly are you seeing in the Zapruder film? It is true that Greer turns to his right and looks back at JFK at the time of the head shot, a somewhat suspicious circumstance as Greer claimed he never looked back. Greer then faces front and the car accelerates. Equally suspicious is the fact that, contrary to Secret Service regulations which state that at the first sign of trouble the driver should accelerate out of the area, Greer actually hit the brakes at the time of the first shots bringing the limousine to almost a complete stop. He then looks over his right should at JFK as the head shot(s) are delivered and only then faces front and accelerates.
In those days it was customary for men to wear hair grease and Kellerman's hair was especially slicked down. Watch Kellerman closely and you will see the sunlight glinting off the top of his head. You will also notice that the "pistol" moves simultaneously with Kellerman's head. Also notice a few frames earlier when the sunlight gleams off both the top of Kellerman's head and Greer's. The brightness of the highlight is the same. As Greer turns to his right, Kellerman bends forward slightly and the sun fully catches the top of his head, The resulting bright highlight indeed is a horizontal L shape and could be misidentified as a gun, especially if one has been pre-conditioned to think of it this way. But notice as Kellerman's head moves, so does the shape. The brief flash seen is simply the sunlight reflecting off the metal rim of the bar frame for the privacy window, used when the top was on the car. That day, the top was down and the privacy window rolled down.
I have studied first-generation clear prints of the Zapruder film and it is apparent that while it may be questionable as to why Greer turned and looked back at Kennedy and then could not recall that fact for the Warren Commission, at no time did his two hands leave the steering wheel of the limousine.
-- Jim Marrs, author of Crossfire: The Plot That Killed Kennedy