How did the USSR come to know about the Manhattan Project?

Even though the Soviet Union or USSR as it was known at the time were part of the allied powers with the United States in World War II, they did not know about the building of the nuclear bombs. The Soviets had actually been siding with the Nazis and Japan and were considered to be part of the axis powers until 1941 when they joined the other side.

Therefore, there was not a lot of trust with the Soviet Union when they became a part of the allied powers. The only time that the USSR knew about the nuclear bombs from the U.S. was when the bombs were actually dropped on Japan. It is believed that the USSR was creating nuclear bombs themselves to detonate and put an end to the war, but the U.S. beat them to it.