On 14 September 1939, she arrived in Ravensbrück. In August 1943, she was promoted to the highest rank of Chef Oberaufseherin (Chief senior supervisor), with control and the responsibility of monitoring all of Ravensbrück camp. Klein reached the highest rank that the Nazis allowed a woman in a camp; she received a higher salary, better housing, better food (which was not cooked by detainees, but by other SS women), the best clothes, more power, and this hierarchical title of honour. She oversaw all guards at Ravensbrück until the SS assigned her to the Sachsenhausen concentration camp in September 1944. There, she served as a supervisor, with the same rank until the liberation of the camp by the Allies in April 1945.
For the period of her camp service, August 1943 to August 1944 in Ravensbrück, Klein was freed on 21 July 1948 due to lack of evidence, during the seventh Ravensbrück Trial in Hamburg. (Trans. from French) This trial lasted from 2 July to 21 July 1948. The charges were mistreatment of inmates of Allied nationality and participation in the selection of inmates for the gas chamber.
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