CIA and FBI aid the conspiracy theorists by refusing to release all documents 60 years later.
They can’t release it because it implicates them.
Simple.
Likey all the actors are dead and I’m not sure how much farther the reputations of either agency can fall, but for sure knowing the CIA worked with the Chicago mob to kill a stitting US president would bring unwanted public outcry and “vows to investigate” by the iron spined GOP.