Is it because he said Lee Harvey Oswald was recruited by the KGB while he was in Japan, that he killed JFK, and that putative* KGB officer Yuri "The KGB Had Nothing to Do with Oswald in the USSR" Nosenko was a false defector?
*I say "putative" because KGB true defector Pyotr Deriabin determined that Nosenko didn't know basic KGB things like how to send a cable, how many floors of the American Embassy were dedicated to the CIA (3), and where the cafeteria was in the KGB headquarters building.
