Victims usually arrived at the camps by freight train. Almost all arrivals at the Operation Reinhard camps of Treblinka, Sobibór, and Bełżec were sent directly to the gas chambers, with individuals occasionally selected to replace dead workers. At Auschwitz, camp officials usually subjected individuals to selections; about 25% of the new arrivals were selected to work. Those selected for death at all camps were told to undress and hand their valuables to camp workers. They were then herded naked into the gas chambers. To prevent panic, they were told the gas chambers were showers or delousing chambers. The procedure at Chełmno was slightly different. Victims there were placed in a mobile gas van and asphyxiated, while being driven to prepared burial pits in the nearby forests. There the corpses were unloaded and buried.
Entering conquered Soviet territories alongside the Wehrmacht (the German armed forces) were 3,000 men of the Einsatzgruppen (“Deployment Groups”), special mobile killing units. Their task was to murder Jews, Soviet commissars, and Roma in the areas conquered by the army. Alone or with the help of local police, native anti-Semitic populations, and accompanying Axis troops, the Einsatzgruppen would enter a town, round up their victims, herd them to the outskirts of the town, and shoot them. They killed Jews in family units. Just outside Kiev, Ukraine, in the ravine of Babi Yar, an Einsatzgruppe killed 33,771 Jews on September 28–29, 1941. In the Rumbula Forest outside the ghetto in Riga, Latvia, 25,000–28,000 Jews were shot on November 30 and December 8–9. Beginning in the summer of 1941, Einsatzgruppen murdered more than 70,000 Jews at Ponary, outside Vilna (now Vilnius) in Lithuania. They slaughtered 9,000 Jews, half of them children, at the Ninth Fort, adjacent to Kovno (now Kaunas), Lithuania, on October 28.
From the earliest years of the Nazi regime, German authorities persecuted homosexuals and others whose behavior did not match prescribed social norms. German police officials targeted thousands of political opponents (including Communists, Socialists, and trade unionists) and religious dissidents (such as Jehovah's Witnesses). Many of these individuals died as a result of incarceration and maltreatment.
In 1935, Mengele earned a PhD in anthropology from the University of Munich. In January 1937, he joined the Institute for Hereditary Biology and Racial Hygiene in Frankfurt, where he worked for Dr. Otmar Freiherr von Verschuer, a German geneticist with a particular interest in researching twins. As von Verschuer's assistant, Mengele focused on the genetic factors that result in a cleft lip and palate, or a cleft chin. His thesis on the subject earned him a cum laude doctorate in medicine (MD) from the University of Frankfurt in 1938. (Both of his degrees were revoked by the issuing universities in the 1960s.) In a letter of recommendation, von Verschuer praised Mengele's reliability and his ability to verbally present complex material in a clear manner. The American author Robert Jay Lifton notes that Mengele's published works were in keeping with the scientific mainstream of the time, and would probably have been viewed as valid scientific efforts even outside Nazi Germany.