JOHN YANG: As President Biden leaves office, he's been exercising his presidential clemency powers.

Just this week, he commuted nearly 2,500 nonviolent drug offenders sentences.

It was, he said, an important step toward righting historic wrongs.

He's also being asked to right what some people see as another historic wrong and exonerate Ethel Rosenberg.

She and her husband, Julius, were executed in 1953 after being convicted as Soviet spies in a sensational atomic espionage case at the height of the Cold War Red Scare.

While historians regard Julius Rosenberg as a Soviet spy, his wife's role has been debated for years.

And a document declassified last year is seen as the strongest evidence yet of her innocence.

Robert Meeropol is the younger son of Julius and Ethel Rosenberg.

He and his brother Michael have been working for decades to clear their mother's name.

Mr. Meeropol, first of all, can you tell us about this document?

What is it and what does it say?

ROBERT MEEROPOL, Son of Ethel Rosenberg: Well, what's remarkable about this document is, first of all, who wrote it?

That is Meredith Gardner, who was the chief decryptor, considered to be the heroic spy catcher who broke the Soviet codes, which led to the arrest of certain people.

Now, what he said in this memo about my mother was that she knew about her husband's work, but due to ill health, did not engage in the work itself.

And earlier in the memo, he described her husband's work that my father, Julius Rosenberg, as spying.

So what we have here is a definitive pronouncement by the chief decryptor at the time of my mother's arrest that she was not a spy.

And what's remarkable about this document is it dovetails with earlier work that Meredith Gardner did in decrypting what are called the Venona transcriptions of Soviet spy cables in which it was noted that the KGB gave all of its agents code names, but Ethel had no code name.

JOHN YANG: Armed with this new evidence, what do you want President Biden to do, and where does that process stand as he is about to leave office?

ROBERT MEEROPOL: Well, ideally, we would like President Biden to apologize for the wrongful conviction and execution of my mother.

But the reality is whether you're going to call it an exoneration or a pardon or an apology, any step, major step, that he takes to show that my mother was wrongfully executed would be a remarkable step in the right direction.

And we know that the pardon office has our material, that it is under consideration.

So we're kind of sitting on edge in the last few days of Biden's administration, knowing that we might not hear anything until almost the last minute, but we remain hopeful.

JOHN YANG: I should note that there have been historians who read that document in other ways.

Mark Kramer directs Harvard's "Cold War Studies Project."

He told PBS News the declassified Soviet and U.S. documents make clear that the August 1951 document, that's the document we're Talking about changes nothing.

The Rosenbergs were convicted of conspiring to commit espionage, and both of them were indeed guilty of that crime.

The punishment was unjust, but the guilty verdict was fully justified.

What do you say to that?

ROBERT MEEROPOL: Well, this is a historian who's made his career claiming that Ethel and Julius Rosenberg were both guilty spies.

And when you have a definitive statement that comes out like this from the chief decryptor that says, no, she wasn't a spy.

It is very difficult for them to admit, like, it's difficult for a lot of people that they were wrong, particularly when it's based on their careers.

In fact, it's kind of embarrassing because the reality is what this historian is saying is that he knows better than the KGB who is the KGB agent.

JOHN YANG: Now, this is not the first time or first evidence to suggest your mother's innocence.

Within the past decade, there have been some others.

What else has come up?

ROBERT MEEROPOL: This particular memo is kind of like the final piece of a jigsaw puzzle that my brother and I have been putting together for 50 years.

We filed our first Freedom of Information Act lawsuit in the mid-1970s, and one of the first set of documents we got were Justice Department and FBI documents saying the case against Ethel Rosenberg was weak, but we should arrest her so we can use her as a, quote, lever to force her husband to cooperate.

Then we find out that at the trial, the only people who testified against my mother, David and Ruth Greenglass, gave oral testimony.

There's no physical evidence, and it turns out that they swore the opposite before the grand jury.

David Greenglass said he never even talked to my mother about spying.

So we know that the only evidence presented against her was by proven perjurers.

Now that said Judge Coffman, in sentencing her to death, said she was a full-fledged participant in this crime.

And yet a mother of two young children ends up executed for something that she didn't do.

JOHN YANG: Mother of two young children.

You and your brother were quite young when your parents were arrested.

What are your memories?

ROBERT MEEROPOL: Well, you know, I was three at the time of my parents arrest, so I don't really have many specific memories of that.

Most of my memories come from visiting them in prison.

And they were kind of calm affairs.

They weren't, what, hysterical.

They wanted me to think things were normal and I wanted them to be normal.

So they fooled me and I was easy to fool.

JOHN YANG: What would a presidential apology mean to you and your brother?

ROBERT MEEROPOL: It would mean a tremendous amount.

It's not only a personal thing for us.

People could understand why the children of people convicted of this kind of crime would try to clear their names for personal reasons.

But this is more than personal.

The reason this travesty occurred and my mother was wrongfully executed was because of the politicization of the Justice Department and the American judicial system during the Great Red Scare of the early 1950s.

We now face a very similar situation where the Justice Department and the rule of law are under threat.

So this couldn't be more timely to show how the judicial process can be abused.

It will help to prevent it from happening in the future.

JOHN YANG: Robert Meeropol, one of the sons of Ethel Rosenberg.

Thank you very much.

ROBERT MEEROPOL: Thank you.