This is the report that General Groves, head of the Manhattan Project, sent to the Secretary of War, Henry Stimson, after the first test of the atomic bomb. This was itself then conveyed to Truman. Truman also approved the press release to be sent out prior to the bomb being dropped.

Prior to this, Truman had been briefed by Secretary of War Stimson and Groves several times about the atomic bombs (the first time being the day that Roosevelt died), though it is worth remembering that prior to the Trinity test the power of the bomb was only spoken of in somewhat vague estimates and not nearly as evocatively as after the test.