It was the most notorious spy case of the Cold War — the conviction and execution of Julius and Ethel Rosenberg for passing atomic secrets to the Soviet Union — and it rested largely on the testimony of Ms. Rosenberg’s brother David Greenglass, whose name to many became synonymous with betrayal.

For his role in the conspiracy, Mr. Greenglass, an Army sergeant who had stolen nuclear intelligence from Los Alamos, N.M., went to prison for almost a decade, then changed his name and lived quietly until a journalist tracked him down. He admitted then, nearly a half-century later, that he had lied on the witness stand to save his wife from prosecution, giving testimony that he was never sure about but that nevertheless helped send his sister and her husband to the electric chair in 1953.

Mr. Greenglass died on July 1, a family member confirmed. He was 92. His family did not announce his death; The New York Times learned of it in a call to the nursing home where he had been living under his assumed name. Mr. Greenglass’s wife, Ruth, who had played a minor role in the conspiracy and also gave damning testimony against the Rosenbergs, died in 2008.

In today’s world, where spying has more to do with greed than ideology, the story of David Greenglass and the Rosenbergs is an enduring time capsule from an age of uncertainties — of world war against fascism, Cold War with the Soviets, and shifting alliances that led some Americans to embrace utopian communism and others to denounce such ideas, and their exponents, as un-American.