In this Aug. 6, 1945 photo released by the U.S. Army, a mushroom cloud billows about one hour after a nuclear bomb was detonated above Hiroshima, Japan.

The ground crew of the B-29 "Enola Gay" which atom-bombed Hiroshima, Japan on August 6, 1945 poses for a photo with the aircraft at their base in Tinian, Mariana Islands in this undated U.S. Air Force handout image.

The Boeing B-29 Superfortress "Enola Gay" lands at the Tinian airbase in the Mariana Islands after the atomic bombing mission on Hiroshima, Japan in this U.S. Air Force handout photo dated August 6, 1945.

In this Aug. 8, 1945 file photo, soldiers and civilians walk through the grim remains of Hiroshima, two days after the atomic bomb explosion of Aug. 6,1945. The building on left with columned facade was the Hiroshima Bank. To its right, with arched front entrance, was the Sumitomo Bank. On Aug. 6, 1945, a U.S. plane dropped an atomic bomb on Hiroshima, the first nuclear weapon has been used in war. Japan surrendered on Aug. 15, ending World War II.

This general view of the city of Hiroshima showing damage wrought by the atomic bomb was taken March 1946, six months after the bomb was dropped August 6, 1945.

The Hiroshima Prefectural Industrial Promotion Hall, currently called the Atomic Bomb Dome or A-Bomb Dome, is seen in Hiroshima, Japan, before (top) and after (bottom) the August 6, 1945 atomic bombing of Hiroshima, in this combination image of handout photos released by Hiroshima Peace Memorial Museum. The top image is undated and the bottom image was taken by Shigeo Hayashi between October 1 and 10, 1945.

The Hiroshima Prefectural Industrial Promotion Hall, currently called the Atomic Bomb Dome or A-Bomb Dome, is seen from Aioi Bridge in Hiroshima before (top) and after (bottom) the August 6, 1945 atomic bombing of Hiroshima, Japan, in this combination image of handout photos released by the Hiroshima Peace Memorial Museum. The top image is undated and the bottom image was taken by Shigeo Hayashi between October 1 and 10, 1945.

Aerial pictures of Hiroshima, Japan, taken in April 1945 before the atomic bomb was dropped and, in August 1945 after the bombing, show the extent of the devastation on the city.

In this Aug. 6, 1945, file photo, survivors are seen as they receive emergency treatment by military medics shortly after the first atomic bomb ever used in warfare was dropped by the United States over Hiroshima, western Japan. Many people exposed to radiation developed symptoms such as vomiting and hair loss. Most of those with severe radiation symptoms died within three to six weeks. Others who lived beyond that developed health problems related to burns and radiation-induced cancers and other illnesses.

The shell of an apartment building 2,600 feet from ground zero in Hiroshima is all that is left following the dropping of the atom bomb on this Japanese city in 1945.

A view of the total destruction of Hiroshima, the result of the first atomic bomb dropped in wartime, August 6, 1945.

Smoke rises more than 60,000 feet into the air over Nagasaki from an atomic bomb, the second ever used in warfare, dropped from a B-29 Superfortress bomber in this U.S. Air Force handout photo dated August 9, 1945.