Which sentence or phrase best supports the idea that a bomb has dropped

The very first sentence of Hiroshima ells the reader that the bomb has been dropped.

At exactly fifteen minutes past eight in the morning, on August 6, 1945, Japanese time, at the moment when the atomic bomb flashed above Hiroshima, Miss Toshiko Sasaki, a clerk in the personnel department of the East Asia Tin Works, had just sat down at her place in the plant office and was turning her head to speak to the girl at the next desk.