This section seems relevant:

At first, some refused to believe the United States had built an atomic bomb. The Japanese Army and Navy had their own independent atomic-bomb programs and therefore the Japanese understood enough to know how very difficult building it would be.[74] Admiral Soemu Toyoda, the Chief of the Naval General Staff, argued that even if the United States had made one, they could not have many more.[75] American strategists, having anticipated a reaction like Toyoda's, planned to drop a second bomb shortly after the first, to convince the Japanese that the U.S. had a large supply.[59][76]

Emphasis mine.