The operational command of "Einsatzgruppe" B, attached to the Army Group Center, was established under the command of Arthur Nebe a few days after the German attack on the Soviet Union in Operation Barbarossa. "Einsatzgruppe" B departed from the occupied city of Poznań (Posen) on 24 June 1941, with 655 men from the Security Police, Gestapo, Kripo, SD, Waffen-SS and the 2nd Company of Reserve Police Battalion 9. On 30 June 1941 Himmler visited the newly formed Bezirk Bialystok district and pronounced that more forces were needed in the area, due to potential risks of partisan warfare. The chase after the Red Army's rapid retreat left behind a security vacuum, which required urgent deployment of additional personnel.