Lee Harvey Oswald's lost year in the USSR
I believe Oswald was the lone shooter.
But the fact there were no witnesses to that lost year and that the KGB agent Nosenko was sent as a disinformation officer make me think there was at least SOME Soviet influence on Oswald.
Soviets were the best spies along with the British but I think the Soviets had the edge since they managed to get Kim Philby in as a mole.
James J. Angleton probably went to his grave without the answer to this mysterious lost year of Oswald's.
"Encerrarlo, y no habla."