In the 1940's Wallenberg had taken a job with Stockholm - based food - exporting company. It's owner that was a Jew, could no longer travel safely due to Nazis and the dictatorship of Adolf Hitler through out Europe.
Wallenberg replace him on these trips, making him acquainted with the capital of Hungary, Budapest.
In January 1944 Nazis occupied Hungary which was home to the last sizable Eastern and Central European Jewish population. The government was planning to obliterate all European Jews.
The War Refugee Board in the United States requested an envoy ( or messenger ) from Sweden, which had stayed neutral throughout the World War 2, to make a rescue effort.
Wallenberg was selected to be that envoy.
During the following month of July in Budapest, Wallenberg's office provided protection passports and hideouts to thousands of Jews.
According to Gromyko, it was revealed in recently discovered paperwork that he had succumbed to heart disease and was cremated in Moscow's Lubyanka prison in July 1947.
The paperwork was never handed over to Swedish authorities, nor an explanation of why he was incarcerated.