 Death camps differed from concentration camps located in Germany proper, such as Bergen-Belsen, Oranienburg, Ravensbrück, and Sachsenhausen, which were prison camps set up prior to World War II for people defined as 'undesirable'.From March 1936, all Nazi concentration camps were managed by the "SS-Totenkopfverbände" (the Skull Units, SS-TV), who operated extermination camps from 1941 as well.An SS anatomist, Dr Johann Kremer, after witnessing the gassing of victims at Birkenau, wrote in his diary on 2 September 1942