 At the request of Norris Bradbury, who had replaced Robert Oppenheimer as director of the Los Alamos Laboratory in October 1945, Fuchs stayed on at the laboratory into 1946 to help with preparations for the Operation Crossroads weapons tests. The US Atomic Energy Act of 1946 (McMahon Act) prohibited the transfer of information on nuclear research to any foreign country, including Britain, without explicit official authority, and Fuchs supplied highly classified US information to nuclear scientists in Britain as well as to his Soviet contacts. As of 2014 British official files on Fuchs were still being withheld.
