The killing centres had no electric fences, as the size of the prisoner "Sonderkommandos" (work units) remained relatively easy to control – unlike in camps such as Dachau and Auschwitz. To assist with the arriving transports only specialised squads were kept alive, removing and disposing of bodies, and sorting property and valuables from the dead victims. The "Totenjuden" forced to work inside death zones were kept in isolation from those who worked in the reception and sorting area. Periodically, those who worked in the death zones would be killed and replaced with new arrivals to remove any potential witnesses to the scale of the mass murder.