What were the concentration camp guards called?

Answer 1

Nazi

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Most of them were members of the SS, either from the 'Death's Head' Battalion or from one of the foreign battalions.

There was no special name, they would be called by 'rank, surname'.

Answer 2

Yes, they were called Kapos - they were inmates themselves, but acted as trustees to avoid being killed themselves. At least one surviving Kapo has been prosecuted as a war criminal.

Answer 3

They were called Kapos (Capos).