This matter was briefly considered by the Interim Committee, which was composed of scientists and statesmen, headed by the Secretary of War, Henry Stimson. They deferred to a Scientific Committee composed of four physicists, who rejected the idea. The suggestion was never passed any further; Truman was never consulted on this question, for example, or told that it was an option. See this blog post of mine from awhile back.

I would just note as well that the goal was not, at all, to minimize casualties. The goal was to shock the Japanese into surrender, and to show the world the power of atomic weapons. This is why they went the exact opposite of this approach: they sought urban areas that had not been previously bombed that would showcase the damage. I only bring this up because this kind of question seems to suppose that they were trying to minimize lives taken — they were absolutely not.