During the Manhattan Project time period, were people able to detect the first atomic bomb detonation in New Mexico July 16, 1945? If so, how did they react, seeing as at that time nobody should have had any idea what it was?
Civilians in the surrounding area were aware that a large explosion had occurred. The military put out a press release that claimed that a large ammunition dump had exploded in the desert with no fatalities. This is obviously a bit thin, but the secrecy around the test only had to last a month or so.
The Kodak company was apparently aware that there had been a nuclear test somewhere on US early on because the fallout contaminated packing material used with the film. A 1949 paper identified the contamination as cerium-141, and speculates that the most likely source of contamination was the bomb test.