Command based in the Marianas. By July, the bombers were established at North Field on Tinian, which had just been completed for the 313th Bombardment Wing. It was, however, under the direct operational control of the commander, Twentieth Air Force. The mission of the unit was the operational use of the Atomic Bomb. It had only one Bombardment Squadron—the 393rd, commanded by Major Charles W. Sweeney. The 509th Composite Group was a completely self-sufficient unit, with its own engineer, material, and troop squadrons as well as its own military police unit. Since the Manhattan project was carried out in an atmosphere of high secrecy, the vast majority of the officers and men of the 509th Composite Group were completely ignorant of its intended mission. With the testing of the Atomic Bomb completed in the United States, the two other bombs (Little Boy, Fat Man) had arrived on Tinian on 26 July, being delivered by the . On 24 July, a directive was sent to General Carl A. Spaatz ordering the 509th to deliver its first atomic bomb as soon as weather would permit. The Japanese cities of Hiroshima, Kokura, Niigata and Nagasaki were potential targets. President Harry S. Truman gave his final go-ahead from the Potsdam Conference on 31 July. On 6 August the atomic attack began with a flight of three special reconnaissance F-13As (RB-29s) which took off to report the weather over the primary and secondary targets. Col. Tibbets followed in his B-29 aircraft, Enola Gay, an hour later, accompanied by two other B-29s which would observe the drop. While on the way to Japan, Major Claude Eatherly, flying Straight Flush, radioed that Hiroshima was clear for a visual bomb drop. Navy weapons expert Captain William Parsons armed the bomb while in flight, as it was deemed too dangerous to do this on the ground at North Field, lest an accident happen and the bomb go off, wiping out the entire base. At 8:15 am, the Enola Gay released Little Boy from an altitude of . The radar fuse on the bomb had been preset to go off at an altitude of above the ground. In the ensuing explosion, yielding about 12 kilotons of TNT in explosive power, about 75,000 people were killed and 48,000 buildings were destroyed. With no official statement from the Japanese government, there was no let-up with the conventional B-29 raids. B-29s from the