Roma were among the groups singled out on racial grounds for persecution by the Nazi regime. Roma were subjected to internment, deportation, and forced labor, and were sent to extermination camps. Mobile killing squads (Einsatzgruppen) also killed tens of thousands of Roma in the German-occupied eastern territories. The fate of the Roma closely paralleled that of Jews. It is difficult to determine the number of Roma killed during the Holocaust. It is estimated that of the approximately one million Roma living in Europe before the war, between 250,000 and 500,000 were killed.