During the invasion of Poland, which started World War II, in September 1939, the Germans carried out mass arrests of local Polish activists and banned Polish organizations, and the city was made the headquarters of the southern district of the "Selbstschutz", whose task was to persecute Poles. For most of the war, the fighting did not affect the city. During the war, the Germans opened the graves of medieval Polish monarchs and local dukes to carry out anthropological research for propaganda purposes, wanting to demonstrate German "racial purity". The remains were transported to other places by the Germans, and they have not been found to this day. In 1941 the remnants of the pre-war Polish minority in the city, as well as Polish slave labourers, organised a resistance group called Olimp. The organisation gathered intelligence, carrying out sabotage and organising aid for Polish slave workers. In September 1941 the city's 10,000 Jews were expelled from their homes and soon deported to concentration camps. Few survived the Holocaust. As the war continued, refugees from bombed-out German cities, and later refugees from farther east, swelled the population to nearly one million, including 51,000 forced labourers in 1944, and 9,876 Allied PoWs. At the end of 1944 an additional 30,000–60,000 Poles were moved into the city after the Germans crushed the Warsaw Uprising.