According to historian Gar Alperovitz, the US had intercepted...

According to historian Gar Alperovitz, the US had intercepted cables on the 12th and 13th of July 1945 that indicated the Japanese Emperor wanted to end the war.
Source: Alperovtiz, Gar (1995), The Decision to use the atomic bomb and the architecture of an American myth, New York: Knopf

As early as 1965, Alperovitz wrote "Although Japanese peace feelers had been sent out as early as September 1944 (and [China's] Chiang Kai-shek had been approached regarding surrender possibilities in December 1944), the real effort to end the war began in the spring of 1945. This effort stressed the role of the Soviet Union ...

In mid-April [1945] the [US] Joint Intelligence Committee reported that Japanese leaders were looking for a way to modify the surrender terms to end the war. The State Department was convinced the Emperor was actively seeking a way to stop the fighting."
Source: Alperovtiz, Gar (1965), Atomic diplomacy: Hiroshima and Potsdam - The use of the atomic bomb and the American confrontation with Soviet Power, New York: Simon & Schuster