It was clear to many scientists at Columbia that they should try to detect the energy released in the nuclear fission of uranium from neutron bombardment. On 25 January 1939, a Columbia University group conducted the first nuclear fission experiment in the United States, which was done in the basement of Pupin Hall. The experiment involved placing uranium oxide inside of an ionization chamber and irradiating it with neutrons, and measuring the energy thus released. The next day, the Fifth Washington Conference on Theoretical Physics began in Washington, D.C., under the joint auspices of The George Washington University and the Carnegie Institution of Washington. From there, the news on nuclear fission spread even further, which fostered many more experimental demonstrations.
Bohr and Wheeler overhauled the liquid drop model to explain the mechanism of nuclear fission, with conspicuous success. Their paper appeared in Physical Review on 1 September 1939, the day Germany invaded Poland, starting World War II in Europe. As the experimental physicists studied fission, they uncovered more puzzling results. George Placzek  asked Bohr why uranium fissioned with both very fast and very slow neutrons. Walking to a meeting with Wheeler, Bohr had an insight that the fission at low energies was due to the uranium-235 isotope, while at high energies it was mainly due to the far more abundant uranium-238 isotope. This was based on Meitner's 1937 measurements of the neutron capture cross-sections. This would be experimentally verified in February 1940, after Alfred Nier was able to produce sufficient pure uranium-235 for John R. Dunning, Aristid von Grosse and Eugene T. Booth to test.
Other scientists resumed the search for the elusive element 93, which seemed to be straightforward, as they now knew it resulted from the 23-minute half-life. At the Radiation Laboratory in Berkeley, California, Emilio Segrè and Edwin McMillan used the cyclotron to create the isotope. They then detected a beta activity with a 2-day half-life, but it had rare-earth element chemical characteristics, and element 93 was supposed to have chemistry akin to rhenium. It was therefore overlooked as just another fission product. Another year passed before McMillan and Philip Abelson determined that the 2-day half-life element was that of the elusive element 93, which they named "neptunium". They paved the way for the discovery by Glenn Seaborg, Emilio Segrè and Joseph W. Kennedy of element 94, which they named "plutonium" in 1941.
Another avenue of research, spearheaded by Meitner, was to determine if other elements could fission after being irradiated with neutrons. It was soon determined that thorium and protactinium could. Measurements were also made of the amount of energy released. Hans von Halban, Frédéric Joliot-Curie and Lew Kowarski demonstrated that uranium bombarded by neutrons emitted more neutrons than it absorbed, suggesting the possibility of a nuclear chain reaction. Fermi and Anderson did so too a few weeks later. It was apparent to many scientists that, in theory at least, an extremely powerful energy source could be created, although most still considered an atomic bomb an impossibility.