The New York Times reported Thursday that Sobell, who served nearly 19 years in Alcatraz and other federal prisons, said he and Julius Rosenberg were part of a conspiracy that delivered to the Soviets classified military information.
Among secrets that were shared was what the U.S. government called the secret to the atomic bomb.
When asked if he had shared military secrets with the Soviet Union during World War II, Sobell, 91, told the newspaper he had done so.
"Yeah, yeah, yeah, call it that," he said. "I never thought of it as that in those terms.
"What I did was simply defensive, an aircraft gun," he added. "This was defensive. You cannot plead that what you did was only defensive stuff, but there's a big difference between giving that and stuff that could be used to attack our country."
He said Ethel Rosenberg was "aware of her husband's espionage, but did not actively participate."
The Rosenbergs were executed in 1953 after they were found guilty of conspiracy to commit espionage.
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