A Moscow Court has upheld a decision by the Federal Security Service (FSB) not to declassify prison records which could shed light on the death of Swedish diplomat Raoul Wallenberg.

While working at the Swedish Embassy in Budapest, Wallenberg helped rescue thousands of Jews during World War II. But after the Red Army entered Hungary, he disappeared.

He is believed to have died in 1947 in a prison at the KGB’s Lubyanka headquarters in central Moscow where he was being held on espionage charges.

His relatives have for decades sought to shine light on the circumstances of his death, but have been refused access to prison records.

Wallenberg’s niece Marie Dupuy last summer sued the FSB, the KGB’s successor, for refusing to open prison records, but the case was dismissed by a district court.