The discovery of the neutron by James Chadwick in 1932, followed by that of nuclear fission in uranium by the German chemists Otto Hahn and Fritz Strassmann in 1938, and its theoretical explanation (and naming) by Lise Meitner and Otto Frisch soon after, opened up the possibility of a controlled nuclear chain reaction with uranium. At the Pupin Laboratories at Columbia University, Enrico Fermi and Leo Szilard began exploring how this might be achieved. Fears that a German atomic bomb project would develop atomic weapons first, especially among scientists who were refugees from Nazi Germany and other fascist countries, were expressed in the Einstein-Szilard letter to the President of the United States, Franklin D. Roosevelt. This prompted Roosevelt to initiate preliminary research in late 1939.