As if cold-blooded murder of millions of Jews was not enough, it was done with extreme, perverse cruelty. The victims were packed into cattle trains with standing-room only and without food or water, or heat in the winter, or toilet facilities. Many did not arrive at the camps alive. Those who did arrive at their destination had their heads shaved, with the hair to be used for stuffing mattresses. Stripped of all clothing, most were herded naked into the gas chambers. Dr Johann Kremer, a German SS surgeon, described the gassing of a group of French Jews including 150 boys and girls under the age of 15:
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.