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. During the war the Germans operated four subcamps of the Gross-Rosen concentration camp in the city. Approximately 3,400–3,800 men were imprisoned in three subcamps, among them Poles, Russians, Italians, Frenchmen, Ukrainians, Czechs, Belgians, Yugoslavs, Chinese, and about 1,500 Jewish women were imprisoned in the fourth camp. Many prisoners died, and the remaining were evacuated to the main camp of Groß-Rosen in January 1945. There were also three subcamps of the Stalag VIII-B/344 prisoner-of-war camp, and two Nazi prisons in the city, including a youth prison, with multiple forced labour subcamps. In 1945, the city became part of the front lines and was the site of the brutal Siege of Breslau. Adolf Hitler had in 1944 declared Breslau to be a fortress (Festung), to be held at all costs. An attempted evacuation of the city took place in January 1945, with 18,000 people freezing to death in icy snowstorms of weather. In February 1945, the Soviet Army approached the city and the German Luftwaffe began an airlift to the besieged garrison. A large area of the city centre was demolished and turned into an airfield by the defenders. By the end of the three-month siege in May 1945, half the city had been destroyed. Breslau was the last major city in Germany to surrender, capitulating only two days before the end of the war in Europe. Civilian deaths amounted