Sonderkommando
Because they knew so much about how the Nazis were killing so many people, the Nazis thought of the Sonderkommando as Geheimnisträger — people who knew secrets.  Because of this, they were kept apart from other prisoners in the camps.  The Nazis also did not want anyone outside the camps to know what they were doing.  To make sure the Sonderkommando could never tell what they knew, the Nazis would regularly kill all of the Sonderkommando, usually about every 3 months.  Then they would choose a new group out of new prisoners just getting to the camps.  The new group's first job would be to burn the bodies of the old Sonderkommandos.