A Russian court on Monday rejected a request to release classified documents about Swedish diplomat Raoul Wallenberg, whose death in a Soviet secret police prison seven decades ago remains shrouded in mystery.

Wallenberg used his diplomatic powers to help nearly 20,000 Jews flee Nazi-occupied Hungary during World War Two, before he was detained by Soviets in Budapest before the end of the war.

Read more:Raoul Wallenberg: The Swedish Oskar Schindler

He died in a Soviet prison in Moscow in 1947 from what police officials later said was a heart attack. His body was never handed over and Sweden only declared him dead in 2016.

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Sooner or later the truth will come out

The humanitarian's relatives believe he may have murdered by the Soviet secret police. In 2000, the head of a Russian investigative commission said Wallenberg backed up that assertion.

Wallenberg's relatives in July launched a case against the Russian FSB, the successor to the Soviet-era KGB and NKGB, in order to force the intelligence service to release classified documents that could shed light on the diplomat's fate.

But the Moscow court sided with the FSB argument that the documents could not be released because they contain information about other individuals.

"The historical significance of Raoul Wallenberg gives us the right to know the truth, whatever it is. And sooner or later we will get hold of the truth," said the family's lawyer Ivan Pavlov, adding the decision would be appealed.

The FSB said the family would have to wait until 2022 when a 75-year waiting period for classified documents has passed. FSB representative Sergei Churikov admitted that Wallenberg's files had been heavily censored and parts were missing. "Someone took measures to hide information," he said.

cw/kl (AFP, AP, dpa, Reuters)