In 1939, American scientists, many of whom had fled from fascist regimes in Europe, were aware of advances in nuclear fission and were concerned that Nazi Germany might develop a nuclear weapon. The physicists Leo Szilard and Eugene Wigner persuaded Albert Einstein to send a letter to U.S. President Franklin D. Roosevelt warning him of that danger and advising him to establish an American nuclear research program. The Advisory Committee on Uranium was set up in response. The beginning of the Manhattan Project can be dated to December 6, 1941, with the creation of the Office of Scientific Research and Development, headed by Vannevar Bush.