Oleg Penkovsky

A senior officer of Soviet military intelligence, the GRU, Penkovsky spied in place for MI6 and the CIA through 1961 and 1962 and photographed around 5,000 secret papers in about 14 months. The British businessman Greville Wynne carried material between him and his handlers. His value peaked in the Cuban Missile Crisis, when American photo analysts judged the first U-2 pictures of Cuba partly by comparing them with material he had supplied. He was arrested in October 1962, as the crisis came to a head, and executed on May 16, 1963, according to the official Soviet announcement. Exactly how he was detected is still not settled in the public record.