STOCKHOLM - A newly found Swedish document shows how the KGB intervened in the early 1990s to stop an investigation into World War II hero Raoul Wallenberg's fate, two U.S.-based researchers said Monday.

The Swedish diplomat, who would have turned 100 this year, is credited with rescuing tens of thousands of Hungarian Jews from the Nazis. He disappeared after being arrested in Hungary by the Soviet Red Army in 1945.

Open gallery view Raoul Wallenberg. Credit: AP

The Russians have said he was executed on July 17, 1947, but unverified witness accounts and newly uncovered evidence suggest he may have lived beyond that date.

Wallenberg researchers were hoping that key pieces of the puzzle would emerge when an international commission was granted access to Soviet prison records as the communist rule was heading toward its end.

But a document from the Swedish Foreign Ministry supports claims that the KGB - the former Soviet secret police and intelligence agency - acted to obstruct that effort, said German researcher Susanne Berger, who consulted a Swedish-Russian working group that conducted a 10-year investigation until 2001.

Open gallery view Nina Lagergren tours a photo exhibition of her brother, the Swedish diplomat Raoul Wallenberg, in Stockholm, Sweden in December 2011. Credit: AP

The September 16, 1991, memorandum from the Swedish Embassy in Moscow cites the former head of the Soviet "Special Archive," Anatoly Prokopenko, as telling Swedish diplomats that the KGB instructed him to stop a search for documents by researchers working for the first International Wallenberg Commission.

Prokopenko also said the KGB wanted copies of all documents the researchers had already viewed, according to the memo, which was made available by Berger. Its authenticity was confirmed by the Foreign Ministry.

Berger said the document was significant because it illustrates how since the end of the Cold War researchers have struggled to get access to crucial documents from Soviet archives.

"The action in 1991 has, unfortunately, proved symptomatic, rather than an exception to the rule," Berger said. "Twenty years later, we are still facing this fundamental problem."

Prokopenko said the researchers had been euphoric when they found an archive document on Wallenberg's transfer from one Soviet prison to another, sharing their discovery with other members of the commission investigating Wallenberg's fate.

That was a mistake, the archivist implied, saying the KGB officers on the panel reacted quickly, warning authorities, and Prokopenko was immediately ordered to bar the researchers' access to the files.

Prokopenko said he complied because he was working to open the archives to the public, taking advantage of Soviet leader Mikhail Gorbachev's liberal reforms, and realized that open disobedience would lead to his immediate ouster. "I had to make a sacrifice for the sake of uncovering numerous other secrets of the archive," Prokopenko said.

The Swedish government declassified parts of the memo after Prokopenko mentioned the KGB interference in a 1997 article in a Russian newspaper, but it didn't become publicly known until Berger obtained it this month.

Birstein and Berger, who are based in the U.S., said that though they and other researchers have been granted access to study some Wallenberg files, important archive material has still not been made available.

"At the key junctures, the doors have remained closed," Berger said, noting that even the first piece of material that was handed over by the Russians in 1991, and was meant to illustrate a new openness on their side, turned out to be censored.

It concerned interrogation material suggesting that Wallenberg had been questioned six days after his alleged death.