"Always remember, we were innocent," Julius and Ethel Rosenberg wrote to their two sons in one of their last letters from prison.
The couple were executed in New York's Sing Sing prison in 1953
They were executed on 19 June 1953, the only ones of a group of alleged atomic spies in the United States to be given the death penalty.
Their two sons, then aged 10 and six, were adopted by the Meeropol family, allowed to grow up hidden from public view and protected from the notoriety of their parents.
As adults, in the early 1970s, they took up their legacy - to try to prove their parents' last words true.
Then, earlier this month, one of the Rosenbergs' co-defendants admitted that he and Julius Rosenberg had spied for the Soviet Union. Morton Sobell had denied espionage for 50 years, but in a newspaper interview with the New York Times, he acknowledged that he had passed on military information.
Both brothers accepted what he said.
I asked Robert Meeropol how he could reconcile Sobell's admission with his parents' message.
It depends upon what they were innocent of," he said. "They were killed for stealing the secret of the atomic bomb and they were innocent of that."
But surely, I asked, as children, the two boys had clung to the idea that their parents were unconditionally innocent.
"Yes, that's absolutely correct," he said. "In the 1970s, when my brother and I embarked upon our effort to get all the secret files released we said: 'On the one hand whatever comes out comes out, and that's more important than any particular belief we have. But on the other hand we believe it will prove their innocence'."
He said that the latest information had come at the end of an "evolution" in his thinking though, which prepared him for Sobell's confession. In the mid-1990s the transcripts of secret Soviet transmissions, the "Venona" traffic, were released, showing that Julius Rosenberg had worked for the Soviets.
But Robert Meeropol still believes that even if his father did pass on military information during the war, that was not a crime for which he should have received the death penalty.
Robert Meeropol does not believe his parents should have received the death penalty
At the same time as Morton Sobell admitted espionage, the US authorities finally released transcripts of evidence given to the Grand Jury in the Rosenbergs' case.
Included was the statement of Ruth Greenglass, married to Ethel Rosenberg's brother, and the key witness against her. It was Ruth Greenglass who testified in court that Ethel had typed up handwritten notes about the atom bomb. Ruth Greenglass had made those notes herself: her husband David Greenglass worked at Los Alamos. It was Ethel's typing which implicated her in the conspiracy, and eventually sent her to the electric chair.
In the Grand Jury statement, Ruth Greenglass made no mention of Ethel Rosenberg being present at any meetings, let alone typing up any notes. At the trial, these notes were said to describe the secret of the atomic bomb. Robert Meeropol said according to the Grand Jury statement the notes were only about the buildings on the site.
"You can call that atomic espionage," he said. "But it's certainly not giving away any significant secrets."
You can call that atomic espionage but it's certainly not giving away any significant secrets
To him, this Grand Jury statement confirms that evidence was fabricated for his mother's trial, and that there is no significant evidence against her. He would like to see David Greenglass's Grand Jury testimony too: but Mr Greenglass is still alive, and refuses to let that transcript be released.
Beyond that, there is still a mass of information about the Rosenbergs which is still secret - held by both the Russians and the Americans.
"I think it will be difficult to get entirely to the bottom of this," said Mr Meeropol.
"I think that for years to come, your perspective on this case will tell people just as much about what your political orientation is and your view of the world in general is as it tells you about the case."
It's certainly been true for many years that the Rosenbergs have been a touchstone.
But now it's clear that Julius Rosenberg did spy for the Soviet Union, while his wife appears to be quite innocent, people's views may well be more complex.
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