The prospect of Germany developing an atomic bomb was also of great concern to scientists in the United States, particularly those who were refugees from Nazi Germany and other fascist countries. In July 1939, Leo Szilard and Albert Einstein had written a letter warning the President of the United States, Franklin D. Roosevelt, of the danger. In response, Roosevelt created an Advisory Committee on Uranium in October 1939, chaired by Lyman Briggs of the National Bureau of Standards. Research concentrated on slow fission for power production, but with a growing interest in isotope separation. On 12 June 1940, Vannevar Bush, the president of the Carnegie Institution of Washington, and Harry Hopkins, a key advisor to the president, went to the president with a proposal to create a National Defense Research Committee (NDRC) to co-ordinate defence-related research. The NDRC was formally created on 27 June 1940, with Bush as its chairman. It absorbed the Advisory Committee on Uranium which had gone beyond its original role and was now directing research. It became the Uranium Committee of the NDRC.