The German Democratic Republic's Ministry of the Interior (German: Ministerium des Innern, or MdI) maintained the independent Department of the Alert Units of the Volkspolizei (the country's main law enforcement agency) 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.