HIROSHIMA — A photo discovered Monday at a local elementary school shows the mushroom cloud that rose over Hiroshima after the 1945 atomic bombing split into two portions about half an hour later.

The photo of the cloud, which had separated into an upper and lower portion, was taken on Aug. 6, 1945, from a point about 10 km east of the hypocenter in what is now the town of Kaita, Hiroshima Prefecture. It was snapped about 20 to 30 minutes after the atomic bomb detonated, according to the Hiroshima Peace Memorial Museum.

“Studies by the Imperial navy and others have already discovered that the cloud separated, but the photo confirms it and is thus valuable,” a museum official said.

The photo had appeared in history books about Hiroshima, but the whereabouts of any copy of the photo or the negative was unknown until now, according to the museum.

The photo was found in a collection of about 1,000 articles related to the atomic bombing kept at Honkawa Elementary School in the city of Hiroshima. The materials were contributed by a late survivor, Yosaburo Yamasaki, in or after 1953.

It is not known who took the photo. It will be displayed at a museum located next to the school from this spring.