The industrialization and scale of the murder was unprecedented. Killings were systematically conducted in virtually all areas of occupied Europe—more than 20 occupied countries. Close to three million Jews in occupied Poland and between 700,000 and 2.5 million Jews in the Soviet Union were killed. Hundreds of thousands more died in the rest of Europe. Victims were transported in sealed freight trains from all over Europe to extermination camps equipped with gas chambers. The stationary facilities grew out of Nazi experiments with poison gas during the Aktion T4 mass murder ("euthanasia") programme against the disabled and mentally ill, which began in 1939. The Germans set up six extermination camps in Poland: Auschwitz II-Birkenau (established October 1941); Majdanek (October 1941); Chełmno (December 1941); and the three Operation Reinhard camps, Belzec, Sobibor, and Treblinka, in 1942. A seventh death camp, Maly Trostenets, was established near Minsk in Belarus, then part of the Reichskommissariat Ostland. Discussions at the Wannsee Conference in January 1942 made it clear that the German "final solution of the Jewish question" was intended eventually to include Britain and all the neutral states in Europe, including Ireland, Switzerland, Turkey, Sweden, Portugal, and Spain.
The capture, trial and execution in the early 1960s of Adolf Eichmann, the bureaucratic organizer of the Holocaust, led many people to believe that the Mossad would next want to get its hands on Mengele. Many in Israel and around the world figured that the Mossad would have no trouble doing so. But the truth was that for years, the leaders of the government and the agency were simply not all that interested.
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.