I care for the Japanese people but they have a blind spot about this.
This is a great indicator of just how dense the population is in Tokyo, and all of Japan really. Within a radius of six football fields, they evacuated 16,000 people. Unless this was a High Rise business section, that is an amazing amount of people in such a small space.
Local officials suspect that the bomb fell from a U.S. B29 bomber that crashed after a Japanese fighter plane deliberately hit it in a suicide attack in April 1945.