For Operation Barbarossa (June 1941), initially four "Einsatzgruppen" were created, each numbering 500–990 men to comprise a total force of 3,000. Each unit was attached to an army group: "Einsatzgruppe" A to Army Group North; "Einsatzgruppe" B to Army Group Center, "Einsatzgruppe" C to Army Group South, and "Einsatzgruppe" D to the 11th German Army. Led by SD, Gestapo, and Criminal Police (Kripo) officers, "Einsatzgruppen" included recruits from the regular police (Orpo), SD and Waffen-SS, augmented by uniformed volunteers from the local auxiliary police force. When occasion demanded, German Army commanders bolstered the strength of the "Einsatzgruppen" with their own regular-army troops who assisted in rounding up and killing Jews of their own accord.