Story highlights Raoul Wallenberg was a Swedish envoy based in Budapest in 1944

Diplomat helped save thousands of Hungarian Jews, offering them passports to Sweden

He disappeared in 1945 after Budapest fell to Soviet forces, was never heard from again

(CNN) Raoul Wallenberg, the Swedish diplomat who saved thousands of Jews from certain death during World War II, has been formally declared dead by the Swedish government 71 years after he vanished.

Based in Budapest as a special attaché in 1944, Wallenberg set up shop in the city's Jewish Quarter providing papers to an estimated 20,000 Hungarian Jews, allowing them safe passage to Sweden.

However, the Holocaust hero's fate has been mired in mystery for decades. When Budapest fell to Soviet forces in January 1945, Wallenberg was taken into custody and never seen again.

Nina Lagergren, half-sister of Raoul Wallenberg, holds a picture of her brother in 2015.

Last year, Wallenberg's relatives called on Swedish authorities to declare him dead and provide some semblance of closure for the family. Swedish law dictates that a person must have been missing for at least five years before the tax agency can declare the person deceased.

"Since we believe Raoul Wallenberg went missing end of July 1947, the date of death has been set five years later, which is 31 July 1952," Pia Gustafsson, a legal expert at the Swedish Tax Agency, told CNN in an emailed statement.