Heydrich acted under orders from Reichsführer-SS Himmler, who supplied security forces on an "as needed" basis to the local SS and Police Leaders.[4] Led by SD, Gestapo, and Kripo officers, Einsatzgruppen included recruits from the Orpo, Security Service and Waffen-SS, augmented by uniformed volunteers from the local auxiliary police force.[47] Each Einsatzgruppe was supplemented with Waffen-SS and Order Police battalions as well as support personnel such as drivers and radio operators.[41] On average, the Order Police formations were larger and better armed, with heavy machine-gun detachments, which enabled them to carry out operations beyond the capability of the SS.[47] Each death squad followed an assigned army group as they advanced into the Soviet Union.[48] During the course of their operations, the Einsatzgruppen commanders received assistance from the Wehrmacht.[48] Activities ranged from the murder of targeted groups of individuals named on carefully prepared lists, to joint citywide operations with SS Einsatzgruppen which lasted for two or more days, such as the massacres at Babi Yar, perpetrated by the Police Battalion 45, and at Rumbula, by Battalion 22, reinforced by local Schutzmannschaften (auxiliary police).[49][50] The SS brigades, wrote historian Christopher Browning, were "only the thin cutting edge of German units that became involved in political and racial mass murder."[51]