The family of a Swedish diplomat who saved tens of thousands of Hungarian Jews from the Holocaust is suing the Russian government to find out what happened to him.

Relatives of Raoul Wallenberg, who was last seen being approached by Soviet soldier after the Red Army took Budapest, lodged a lawsuit against Russia’s security service last week, in order to access its files.

The family’s lawyer, Ivan Pavlov, said they “want to force the FSB [the successor to the KGB] to give them access to the originals of the documents,” to finally learn what happened to Wallenberg, who disappeared in 1945 – months before the war in Europe ended – aged 32.

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

Wallenberg, in Hungary as his country’s special envoy, issued more than 9,000 protective passports and sheltered up to 10,000 Jews in 32 buildings he rented and designated as Swedish territory, recruiting almost 350 people to help him.

In 1957, Soviet authorities said he had been jailed in the notorious Lubyanka prison, beside the KGB head office, and that he died of heart failure on 17 July 1947, but the family never believed that. In 2000, a Russian investigative commission said Wallenberg had been shot and killed by KGB officers for political reasons in 1947, but gave no further details. He was officially declared deceased in October last year.