I don’t remember now where I read it, and I don’t care enough to go look it up, but one of the more plausible theories I saw was that one of the SS agents running behind JFK’s car accidentally discharged his weapon (a rifle, if I remember correctly) in the chaos after Oswald began shooting, and that was the bullet that killed JFK. The authorities, knowing what conspiracy theories would arise if it were known that an SS agent’s bullet killed him, to the extent that there could be some massive civil unrest, then covered it all up.
Photos show only one SS guy running to the Kennedy Limo and he certainly did not have a rifle in hand.