Sophia Litwinska made a sworn affidavit that was entered into the British trial of the SS staff at Bergen-Belsen in the fall of 1945. Some members of the SS staff at Belsen had previously worked at Birkenau and they were on trial for crimes committed at both Birkenau and Belsen. One of the men who was tried by the British was Franz Hoessler, the commander of the women's camp at Birkenau in 1942; he was transferred to Bergen-Belsen in December 1944.
Before the war, Mengele had received doctorates in anthropology and medicine, and began a career as a researcher. He joined the Nazi Party in 1937 and the SS in 1938. He was assigned as a battalion medical officer at the start of World War II, then transferred to the Nazi concentration camps service in early 1943 and assigned to Auschwitz, where he saw the opportunity to conduct genetic research on human subjects. His subsequent experiments focused primarily on twins, with little regard for the health or safety of the victims.