An estimated 15 to 20 million people were imprisoned in 42,500 camps and ghettos, and often pressed into slavery during the subsequent years,[7] according to research by the United States Holocaust Memorial Museum conducted more recently.[7] The system of about 20,000 concentration camps in Germany and German-occupied Europe played a pivotal role in economically sustaining the German reign of terror.[2] Most of them were destroyed by the Germans in an attempt to hide the evidence of war crimes and crimes against humanity; nevertheless tens of thousands of prisoners sent on death marches were liberated by the Allies afterward.[8]