On the morning of August 9, 1945, the crew of the American B-29 Superfortress Bockscar, flown by Major Charles W. Sweeney and carrying the nuclear bomb nicknamed "Fat Man," found their primary target, Kokura, to be obscured by clouds. After three runs over the city and having fuel running low due to a fuel-transfer problem, they headed for their secondary target, Nagasaki. At about 07:50 Japanese time, an air raid alert was sounded in Nagasaki, but the "all clear" signal was given at 08:30. When only two B-29 Superfortresses were sighted at 10:53 the Japanese apparently assumed that the planes were only on reconnaissance and no further alarm was given.

A few minutes later, at 11:00, the observation B-29 (The Great Artiste flown by Capt. Frederick C. Bock) dropped instruments attached to three parachutes. These instruments also contained messages to Prof. Ryukichi Sagane, a nuclear physicist who studied with three of the scientists responsible for the atomic bomb at the University of California, urging him to tell the public about the danger involved with these weapons of mass destruction. The letter was not found until after the end of World War II.