It might be noted that, to a surprising extent, much about Hitler’s precise knowledge of the Holocaust remains unclear. For instance, we do not know if Hitler ever saw photographs or newsreels of the killing process, or, indeed, just how comprehensive and brutally frank were Himmler’s reports to Hitler. We have agendas of face-to-face meetings between Hitler and Himmler, at which the Holocaust was to be discussed, but no memorializations or minutes of such meetings. Most of the senior Nazis who were tried at Nuremberg in 1945-1946 (few of whom had any immediate involvement in the killing of Jews) had apparently never seen photographic evidence of the horrors of the concentration camps until their trials; they appeared to be genuinely shocked when newsreels of Belsen and Buchenwald were shown to the court.
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.