In 1951, Morton Sobell was tried and convicted with Julius and Ethel Rosenberg on espionage charges. He served more than 18 years in prison.. Through it all, he maintained his innocence. But on Thursday, Mr. Sobell, 91, dramatically reversed himself. In an interview, he admitted for the first time that he had been a Soviet spy...

Mr. Sobell also concurred in what has become a consensus among historians: that Ethel Rosenberg, who was executed with her husband, was aware of Julius's espionage, but did not actively participate. "She knew what he was doing," he said, "but what was she guilty of?"

Government prosecutors later acknowledged that they had hoped that a conviction and the possibility of a death sentence against Ethel Rosenberg would persuade her husband to confess and implicate others, including some agents known to investigators through secretly intercepted Soviet cables.