How did the Soviet Union gain knowledge of the A-Bomb?

Question

Did they use espionage or have they already known about the A-Bomb before or after US during the Cold War?

Answer 1

espionage. the trials of Julius and Ethel Rosenberg were all about them passing US nuclear secrets to the Russians

Answer 2

They used espionage. Klaus Fuchs, who was working on the Manhattan Project was recruited and he passed details of the A-Bomb on to Soviet agents. He did it from a position that it was scientific information that should be shared rather than from any political ideals.