Ever since he was tried and convicted with Julius and Ethel Rosenberg on espionage charges in 1951, Morton Sobell has maintained his innocence. A U.S. Marshal escorted Morton Sobell, left, to Federal Court in New York in March of 1951. Until now. In an interview on Thursday, Mr. Sobell, who served nearly 19 years in Alcatraz and other federal prisons, 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 vital classified military information and what the American government claimed was the...