 George Abramovich Koval ( Russian : Жорж ( Георгий ) Абра ́ мович Кова ́ ль ; IPA : [ ˈʐorʐ ( ɡʲɪˈorɡʲɪj ) ɐˈbraməvʲɪtɕ kɐˈvalʲ ] , Zhorzh Abramovich Koval , December 25 , 1913 – January 31 , 2006 ) was an American who acted as a Soviet intelligence officer . According to Russian sources , Koval 's infiltration of the Manhattan Project and the Los Alamos National Laboratory was "one of the most important intelligence operations of the Cold War ." [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [