Who is Yuriy Nosenko from Case Closed: Lee Harvey Oswald and the Assassination of JFK and what is their importance?

Thirty-five-year-old, Yuriy Nosenko, was a KGB agent. Shortly after Kennedy's assassination, Nosenko contacted the CIA. His intent was to defect from the Soviet Union. Five long years later, he was released from prison and given security clearance to remain in the United States. In return, Yuriy provided integral information to the CIA about Oswald's defection to Russia.