Given the secrecy around the Manhattan Project in the 1940s, American scientists and historians have never been sure how the Soviet Union managed to join the atomic age as early as it did. Four years after the US set off the first atomic bomb, the Soviets detonated a similar one. The answer might have been found. Three spies at the Los Alamos laboratory—where the bomb was built—were known to have given the Soviets information about that first bomb, but not enough to explain the quick progress. Last fall, a fourth spy was found to have operated at Los Alamos, but the historians' paper said they didn't know much about what secrets he'd passed on. Now, the New York Times reports, newly declassified documents show Oscar Seborer's role could have been much more damaging than those of the other three spies. "We had no idea he was that important," one of the historians said.
Seborer was a pioneer in weapon miniaturization, having helped design the firing circuits for the bomb's detonators. That technology dramatically cut the amount of fuel the bomb needed and was used in developing small, lightweight—but powerful—missile warheads, per the Times. One newly released file showed Seborer attesting to the importance of his role. After moving to the Soviet Union in 1951, he reportedly said he'd be executed if he returned to the US. Another clue to the importance of his role was provided at Seborer's funeral, per Live Science. Among the mourners was one representing the espionage agency that succeeded the KGB. (Read more atomic bomb stories.)