I'm convinced it would have been one
Samuel Byck type or at most one leader cut from the same cloth and a few associates and acquaintances (relatives or lovers, potentially). Any group with the slightest self-preservation instinct knows that they'll instantly become the biggest public enemies (which is a deterrent against going too big even later).
Hang on a sec, one disturbed person.., this is going into soft parallel AH, but what if Lee Harvey Oswald in that timeline carried out such an attack instead of what he did IOTL?