The Secret Service Theory of the JFK assassination, proposed by Howard Donahue, states that Kennedy was shot by two gunmen. Lee Harvey Oswald fired the first shot (which hit the concrete) and the second (which hit Kennedy in the neck). Oswald’s bullets were typical, full metal jacket, drive-through bullets, whereas the third (which hit JFK’s skull) was a fragile one, designed to explode on impact.
This theory states that when JFK was shot in the neck, Secret Service agent, George Hickey, who was riding in the car behind the president’s, took out his rifle, stood up, and as his car lunged forward, lost his balance, and accidentally pulled the trigger.
There’s a photo of Hickey holding the rifle during the period of the assassination. Ten witnesses testified they could smell gun-smoke at street level. This theory also claims that, based on the trajectory, the third bullet came from behind.