The East German Ministry of the Interior maintained the independent Department of the Alert Units of the Volkspolizei known as the Volkspolizei-Bereitschaften (VPB). It consisted of between 12,000 and 15,000 men (sources disagree) in 21 Volkspolizei Alert Units of battalion strength. There was usually one unit per district of East Germany but the key districts of Halle, Leipzig and Magdeburg, with their large working class populations, and Potsdam all had two units.