This person was very famous when I grew up, incidentally because he was the only war hero that Sweden had. He saved about 10,000 Jews in Budapest from being sent to concentration camps. He did that by giving them some kind of temporary Swedish passport, and then sent them to Sweden. Some stayed but most moved to relatives in the USA or Israel.

Yes, it's certain that Wallenberg was taken by Soviets. He was in his early 30s.

They thought he was a spy, either for Germany or possibly America, because he was friends with some the Germans who were there. His family was very rich and he had lots of connections to the USA. The Soviets thought he was suspicious and wanted to question him. Apparently he spoke a bit Russian, and they found this suspicious too.

He was taken to a prison in Moscow, called Ljublanka.
The USSR and Russia has always said that he was shot there, in 1947.

Quote Originally Posted by Swedish Wikipedia
Russian interrogation records from March 1945 and May 1947 show that the accused had given protective Swedish passports to individuals who were on the run from Soviet authorities and anti Soviet elements"[23]
Sweden initially accepted the story that he was dead and did not want to make a big deal out of it since lots of tragic mistakes had taken place in the course of the war, and this appeared to be such a mistake.

However, people started to believe that the USSR was lying about Wallenberg being dead, and that he was in fact alive, in prison. In my childhood I remember reading stories about how he was quite likely alive. By then, he was a big hero because Jews in Israel and the USA wanted to thank him for saving their lives and had started trying to find out information about him.

Several people who had been in prison in Russia and later emigrated said that while in prison, they had spoken to a person who said he was Swedish, who spoke lots of languages and generally fit the description of Wallenberg. No other Swedish citizen who fit this description was missing in Russia, so it was assumed this had to be Wallenberg. But the USSR denied it.

Papers in America ran lots of stories about how he was alive, in the gulag.... etc. etc..
The Swedish press picked up the story too, although they were more restrictive. Diplomats etc were asked to try to get some proof that Wallenberg was dead, like a death certificate etc.

After the Cold War this has been thoroughly investigated, and the story that Wallenberg was shot in 1947 seems to be true. I still can't imagine why the USSR shot an innocent citizen of a neutral country two years after the war ended! And it doesn't make any logical sense that the USSR should lie to say someone is dead and keep them inprisoned instead. Particularly not since the relationship between the USSR and Sweden was quite good for the majority of the Cold War.

I lived very near the house where Wallenberg grew up, as a child!