In 1951, Morton Sobell was tried and convicted with Julius and Ethel Rosenberg on espionage charges. He served more than 18 years in Alcatraz and other federal prisons, traveled to Cuba and Vietnam after his release in 1969 and became an advocate for progressive causes.

Through it all, he maintained his innocence.

But on Thursday, Mr. Sobell, 91, dramatically reversed himself, shedding new light on a case that still fans smoldering political passions. In an interview, he admitted for the first time that he had been a Soviet spy.

And he implicated his fellow defendant Julius Rosenberg, in a conspiracy that delivered to the Soviets classified military and industrial information and what the American government described as the secret to the atomic bomb.

In the interview with The New York Times, Mr. Sobell, who lives in the Riverdale neighborhood of the Bronx, was asked whether, as an electrical engineer, he turned over military secrets to the Soviets during World War II when they were considered allies of the United States and were bearing the brunt of Nazi brutality. Was he, in fact, a spy?