The family of Swedish diplomat Raoul Wallenberg, who saved thousands of Hungarian Jews during the second world war before disappearing when Hungary came under under Soviet rule, are suing Russia’s security service for access to its files, their lawyer said Thursday.

“The relatives of Wallenberg filed the lawsuit at the Meshchansky court in the Russian capital on Wednesday,” their lawyer, Ivan Pavlov, told AFP.

The Wallenberg family “wants to force the FSB [the successor to the KGB] to give it access to the originals of the documents” that concern Wallenberg’s fate, Pavlov said.

He said Wallenberg’s relatives have made many attempts to gain access to the FSB archives dating back to the Soviet era. These were either rejected or the documents they received were incomplete, Pavlov said.

“This case isn’t just about the possibility of restoring the memory of a remarkable person. It is also yet another attempt to fight the inaccessibility of the FSB archives,” he said.

As a special envoy in Nazi-controlled Hungary, Wallenberg issued Swedish identity papers to tens of thousands of Jews, allowing them to flee Nazi-occupied Hungary and likely death.

But when the Soviets entered Budapest in January 1945 – months before the war ended – they summoned Wallenberg to their headquarters. After that he disappeared, aged 32.

In 1957, the Soviet Union released a document saying Wallenberg had been jailed in the Lubyanka prison, the notorious building where the KGB security services were headquartered, and that he died of heart failure on 17 July 1947.

But his family refused to accept that version of events, and for decades have been trying to establish what happened to him.

They specifically want to know if Wallenberg was “Prisoner number 7” who, according to records, was interrogated on 23 July 1947 – six days after Wallenberg’s alleged death.

The family learned of the mysterious prisoner from two historians who said they had been told by FSB archivists the prisoner was likely to have been Wallenberg.

“The majority of our questions revolve around this prisoner,” Wallenberg’s niece, Marie Dupuy, told AFP.

“Every time, they [the Russian authorities] tell us that they are not able to answer. But we are sure they know.”

In 2000, the head of a Russian investigative commission conceded that Wallenberg had been shot and killed by KGB agents in Lubyanka in 1947 for political reasons, but declined to be more specific or to cite hard evidence.

Last year Sweden officially declared Wallenberg dead, but his body has never been returned to his family.