History: Hours Before Atomic Bombing Of Hiroshima, August 6, 1945, United States

70,000–80,000 people, or some 30% of the population of Hiroshima were killed immediately, and another 70,000 injured. Over 90% of the doctors and 93% of the nurses in Hiroshima were killed or injured—most had been in the downtown area which received the greatest damage.
Although the U.S. had previously dropped leaflets warning civilians of air raids on 35 Japanese cities, including Hiroshima and Nagasaki, the residents of Hiroshima were given no notice of the atomic bomb.
• Japanese realization of the bombing
The Tokyo control operator of the Broadcasting Corporation of Japan noticed that the Hiroshima station had gone off the air. He tried to re-establish his program by using another telephone line, but it too had failed. About 20 minutes later the Tokyo railroad telegraph center realized that the main line telegraph had stopped working just north of Hiroshima. From some small railway stops within 16 kilometers (10 mi) of the city came unofficial and confused reports of a terrible explosion in Hiroshima. All these reports were transmitted to the headquarters of the Imperial Japanese Army General Staff.