I've just finished reading [Hiroshima](https://en.wikipedia.org/wiki/Hiroshima_(book) which talks directly about the reports being circulated in the days after the bomb went off.

Hiroshima had largely been left unscathed prior to the bomb and there were rumours that it was being saved for testing a new deadly weapon, but no-one knew what.

After the bomb dropped there was a lot of confusion. There was speculation that originally it was a firebombing, then local newspapers reported that as so many people had experienced the bright llght, similar to a photographer's bulb upon detonation, it was rumoured to be some kind of magnesium dust which the Americans had managed to spray over the city and then detonate. Some doctors had made their own deductions, for example, in basement vault where a hospital kept its X-rays, someone discovered that the X-rays have all been exposed, leading to more speculation and questions.

The Japanese government were checking out the amount of damage and the scientific community were considering what kind of bomb this could have been, but all news was censored and eventually people found out via American Shortwave broadcasts that it was an atom bomb.

The book is great if you are interested in finding out about the days immediately afterwards - there are some book notes here