In June of 1960, KGB officers shot Pyotr Semyonovich Popov in the back of the head after he was abruptly recalled from East Berlin to Moscow. Formerly a field-grade officer in the Main Intelligence Directorate (GRU) of the Soviet military, Popov was the CIA’s most valuable Human Intelligence (HUMINT) asset during the Cold War, providing hundreds of millions of dollars’ worth of military intelligence and the names of over 600 Soviet agents. Despite his invaluable contribution, however, the CIA missed vital clues left by Popov after his arrest by the KGB in 1959 and subsequent use as a double agent. Why did the CIA fail one of its most important sources? Whether through ignorance or complacency, Popov’s handlers had simply become accustomed to the ease of operating with the same seemingly-secure tradecraft that had served them so well in the past.