Kerry was 20 when Kennedy was shot, Oswald was 24.
Maybe they worked as a team, and Kerry was the second shooter.
Actually Kerry was 19 at the time—he was born 12/11/1943, so he was just shy of 20. Curiously, Kerry was almost precisely the same age that Gavrilo Princip was when he shot the Archduke. Does that make Kerry the second shooter?