As the British representative on the American-British-Canadian Council on the sharing of atomic secrets, he was able to provide the Soviet Union with information from Council meetings. This gave Soviet scientists the ability to predict the number of bombs that could be built by the Americans. Coupled with the efforts of Los Alamos-based scientists Alan Nunn May, Klaus Fuchs, and Theodore Hall (who had been identified but was allowed to remain at large), Maclean's reports to his NKVD controller gave the Soviets a basis to estimate their nuclear arsenal's relative strength against that of the United States and Britain. In addition to atomic energy matters, Maclean's responsibilities at the Washington embassy included civil aviation, bases, post-hostilities planning, Turkey and Greece, NATO and Berlin. It has been reported that Maclean suggested to Moscow that the goal of the Marshall Plan was to ensure American economic domination in Europe. Maclean's cover name was "Homer".