Ruth Greenglass: Key witness in the Rosenberg trial

Ruth Greenglass, whose now suspect testimony helped send her sister-in-law to the electric chair, was a key figure in America's most sensational Cold War spy case – culminating in the trial, conviction and still controversial execution of Julius and Ethel Rosenberg in 1953.

Today, as a result of the Venona decrypts of 1940s cable traffic to Moscow from the Soviet Consulate in New York, and previously unavailable Soviet sources, there is scant doubt that Julius (codenamed "Liberal" in the Venona transcipts) was a leader in the plot to pass over information on the Manhattan project to develop an atom bomb. Among his accomplices was David Greenglass, brother of Julius's wife Ethel, who was working as a machinist on the ultra-secret programme at Los Alamos in New Mexico. But, then as now, the evidence of Ethel's involvement was tenuous in the extreme.

None the less, as the US government built the case against Julius, it threatened to bring capital charges of espionage against his wife as well, as a lever to extract a full confession that would reveal details of the ring. But the strategy failed, when Julius refused to co-operate, even though his two young children risked losing not only their father but their mother too. Their bluff called, prosecutors had no choice but to produce evidence showing Ethel was an active participant in the ring. Thanks to Ruth Greenglass, they secured it.

Confronted in 1950, David Greenglass quickly admitted his role as a spy, and agreed to testify against the Rosenbergs to avoid the death penalty. But he offered little that linked Ethel directly with the espionage operation. Ruth, however, did. Still facing indictment herself, she told prosecutors that, one September afternoon at the Rosenberg apartment on the Lower East Side of Manhattan in 1945, she had watched Ethel typing up hand-written notes from her brother about the atom bomb project. When prosecutors asked David Greenglass to confirm Ruth's recollection, he replied that since she had a good memory, she was probably right.

At the Rosenbergs' trial, which began in March 1951, David Greenglass testified that Ethel had been the typist. So, crucially, did Ruth when she was called to the witness stand. Thus could the chief prosecutor proclaim, in his closing statement to the court, that on that September day in 1945, and "on countless other occasions," Ethel "sat at that typewriter and struck the keys, blow by blow, against her own country in the interests of the Soviets". The jury agreed. Both Rosenbergs were found guilty. After their last appeals were rejected by the Supreme Court, they went to the electric chair at Sing Sing prison on 19 June 1953.

If the evidence against Ethel was already slender, it became even more so with publication in 2003 of The Brother: the untold story of the Rosenberg case, in which David Greenglass effectively admitted to the author Sam Roberts that he had lied at the Rosenberg trial.

"I frankly think my wife did the typing. But I don't remember," he is quoted as saying. Then came words that laid bare the divided family loyalties that helped make the Rosenberg case so harrowing. "You know, I seldom use the word 'sister' any more, I've just wiped it out of my mind," he explained to Roberts. "My wife put her in it. So what am I going to do, call my wife a liar? My wife is my wife."

Ruth Printz and David Greenglass were sweethearts from high school days. They married in 1942 when he was 20 and she just 18, just before David was drafted into the military service that would eventually take him to Los Alamos. Both were members of the Communist Party – no crime in itself at a moment when the Soviet Union was an ally of the US against Nazi Germany.

David ultimately served 10 years of a 15-year sentence before leaving prison in 1960. The couple then resumed married life under new assumed identities in the New York metropolitan area, until Ruth's death on 7 April.

The fact only came to light last month when the US federal government agreed to make public grand jury testimony in the Rosenberg case, provided witnesses were either dead or had agreed to the release. In the document, Ruth Greenglass was listed among the dead. Her husband survives her.

Rupert Cornwell

Ruth Leah Printz: born New York 30 April 1924; married 1942 David Greenglass (two children); died 7 April 2008.