This is one of the "half-truths" or "assumptions" that people think are established facts in regard to JFKs assassination. No one knows for a fact that Jackie was "attempting to retrieve parts of JFK’s head". If you watch the Zapruder film, Jackie's face is looking directly into JFK's eyes, her face almost touching his:
Then, JFK's head explodes:
Jackie then panics (as anyone would)
Jackie then heads for the nearest exit:
The car speeds up, and the secret service agent pushes her back into the car for her own safety.
I took these caps from a DVD video called IMAGE OF THE ASSASSINATION that shows the Zapruder film in wide shot, close up and medium shot. If there is a piece of brain or skull on the trunk of the car, it is quite small, and I cannot see it. This DVD shows the Zapruder film clearer than I have ever seen it, and the back of the car looks clean. There certainly is not a great deal of debris on the trunk.
I am not saying it is impossible that she was trying to retrive part of the skull/brain-just that it is not an established fact. Conspiracy nuts start with a theory like this, and then proceed as if it is a proven fact and then attempt to prove something else based on that unproven fact.
Watching the film the first shot seems to come from the fight front, not from the right rear. From the right rear, the scalp piece should have gone into the front seat. But it doesn’t. And Jackie goes immediately over the back seat to grab the disconnected part of Jack’s head. Gruesome, to be sure, but that’s what happens.
If you look closely at the third frame I posted, you will see that JFK's horrific would is consistent with his being shot from behind. The scalp and piece of skull do not go anywhere; they are hanging there, over his face. You can see his ear behind the hanging material. If he were shot from the front, the scalp and skull would be hanging behind his ear.