Einstein, who was concerned that Germany was developing an atomic bomb, wrote to President Franklin Roosevelt urging that the United States accelerate its own work on nuclear weapons to deter the Germans from using any they might develop. Two years later, the United States embarked on the Manhattan Project, its effort to build a nuclear bomb. Though it was Einstein's equation E=mc2 that had shown that the tiny mass of an atom could be converted into a powerful destructive energy, Einstein was never invited to work on the project. The U.S. government distrusted him because of his left–leaning politics by the time Einstein died, the FBI had a nearly 1,500–page file on him. Einstein never expected the atomic bomb to be used. He was horrified when the United States dropped two atomic bombs on Japan, which killed hundreds of thousands of people. "Woe is me," he said, after the first bomb fell on Hiroshima.