Jack and Bobby pissed off a lot of people in the mafia, cia, and fbi. Everyone in LBJ's political circle was under investigation and the Kennedy's were planning to drop him as vice president. But LBJ came from Texas and was buddies with the local politicians and police departments, including the guy who owned the book repository. He had nothing to lose, everything to gain, and people who were willing to help. The "investigation" was a freaking joke, with most physical evidence destroyed immediately, including the car JFK was riding in. LBJ literally appointed the people to run the investigation, led by Dulles, the guy JFK fired after Bay of Pigs. I doubt we'll ever see all of the JFK files, because all they do is support the theory that LBJ and the CIA overthrew the government. Even if everyone directly involved is long dead, the little trust that the public has in the federal government would be gone.