WASHINGTON (AFP).- Grand jury testimony from 1950, unsealed in the United States Wednesday, has called into question the outcome of one of the most sensational spy trials of the Cold War era.

Legal historians say a long-secret transcript suggests Ethel Rosenberg may not have been as deeply involved as her husband Julius Rosenberg in slipping US atom-bomb secrets to the Soviet Union.

Executed in 1953 via the electric chair at New York's notorious Sing Sing prison, the Rosenbergs -- both communists -- remain to this day the only Americans ever executed by their own country for espionage.

"I think everyone agrees that the evidence against Julius was quite formidable," said Georgetown University law professor David Vladeck, who led a legal campaign to unseal the grand jury testimony of Ethel Rosenberg's brother David Greenglass.

"The question is whether Ethel was an active participant in the conspiracy," he told AFP, "and the grand jury material -- we've now reviewed it all -- doesn't really suggest that she was."

Much of the grand jury testimony that led to the indictment of the Rosenbergs for conspiracy to commit espionage has been in the public domain for years.

But until Wednesday, the missing link has been 46 pages of testimony given behind closed doors in 1950 by Greenglass, who died last year at the age of 92.

The star witness at the Rosenbergs' 1951 trial, Greenglass -- who had worked during World War II on the Manhattan Project that developed the atomic bomb -- went on to spend nearly 10 years in prison in a plea deal for conspiracy to spy for the Soviet Union.

On the witness stand, Greenglass claimed he witnessed his sister in the Rosenbergs' apartment in New York in 1945 transcribing atom-bomb secrets on a portable typewriter -- a key detail leading to her conviction.

Never spoke 
But the transcript unsealed Wednesday indicated that Greenglass told the grand jury, at least twice, that he and his sister never spoke "at all" about passing secrets to the Soviets.

"I said before, and say it again, honestly, this is a fact: I never spoke to my sister about this at all," said Greenglass when pressed at one point about Julius Rosenberg asking him in 1945 to stay in the military and keep relaying classified information.

By comparison, Greenglass clearly knew what Julius Rosenberg was doing with the atomic secrets he was passing to him, quoting his brother-in-law as saying: "I am giving it to my friends."

There is also no mention in the testimony of Greenglass telling the grand jury about a hollowed-out wooden table, supposedly provided by the Soviets, that the Rosenbergs allegedly used to put the typewritten notes onto microfilm.

No such table was ever found, and defense lawyers argued the Rosenbergs only had a plain $21 console from Macy's department store, yet it became another key point on their path to the death penalty.

Greenglass himself recanted his testimony in 1996, telling a journalist he had lied under oath about his sister's role in order to protect his wife Ruth, who he believed had typed the notes for the Soviets.

Not much evidence
University of Wisconsin law professor Brad Snyder said the latest document raises questions as to how much federal prosecutors knew about Greenglass changing his story and lying under oath at trial.

"There's not a lot of legal evidence in that transcript against Ethel," said Snyder, who has researched the Supreme Court's failure to take up the possibility of a miscarriage of justice in the case.

"You need an overt act to prove a conspiracy, and there's no overt act in that testimony," he told AFP.

Greenglass's grand jury testimony is posted at nsarchive.gwu.edu, the website of the National Security Archive at George Washington University.