After the invasion, Hitler entrusted the SS with extermination actions codenamed Operation Tannenberg and AB-Aktion to remove potential leaders who could form a resistance to German occupation. The killings were committed by "Einsatzgruppen" (task forces; deployment groups), assisted by local paramilitary groups. Men for the "Einsatzgruppen" units were drawn from the SS, the SD, and the police. Some 65,000 Polish civilians, including activists, intelligentsia, scholars, teachers, actors, former officers, and others, were killed by the end of 1939. When the army leadership registered complaints about the brutality being meted out by the "Einsatzgruppen", Heydrich informed them that he was acting "in accordance with the special order of the Führer." The first systematic mass shooting of Jews by the "Einsatzgruppen" took place on 6 September 1939 during the attack on Kraków.