What happened after the bombing of Hiroshima and Nagasaki in Chapter 31?

In Chapter 31, Hiroshima and Nagasaki were both bombed, but prisoners had no idea of what had happened. The kill all date was approaching. Prisoners were called out and were told the war had ended and to go down to the river to bathe. When they were at the river, a bomber plane flew overhead and flashed a signal in code, telling them the war was over.