Shown in the 1944 photo above, from left to right, are Dr. Josef Mengele, Richard Baer, Karl Hoecker, and Walter Schmidetski. Richard Baer, known as the last Commandant of Auschwitz, was the commander of the main camp; his adjutant was Karl Hoecker. Dr. Josef Mengele was one of 30 SS officers at Auschwitz II, aka Birkenau, who decided who would live and who would die in the gas chambers.

G. Aly, "Final Solution": Nazi Population Policy and the Murder of the European Jews (1999); C.R. Browning (with contributions by J. Matthäus), The Origins of the Final Solution: The Evolution of Nazi Jewish Policy, September 1939- March 1942 (2004); R. Hilberg, The Destruction of the European Jews (20033); P. Longerich, Politik der Vernichtung. Eine Gesamtdarstellung der nationalsozialistischen Judenverfolgung (1998).

12 million people were killed by the nazi party, But there was no trace of written order and many questions still remain unanswered. Half a million Gypsies were murdered by the Nazis. Nazis thought Jews to be as low as Jews. They were worked to death or gassed. Nazis murdered 5-6 million Jews two thirds of European Jewry one third of their population. 55 million perished in all 20 million Soviet citizens 5 million Germans and three million non-Jewish poles. 
German SS and police murdered nearly 2,700,000 Jews in the killing centers either by asphyxiation with poison gas or by shooting. In its entirety, the "Final Solution" called for the murder of all European Jews by gassing, shooting, and other means. Six million Jewish men, women, and children were killed during the Holocaust—two-thirds of the Jews living in Europe before World War II.

Perhaps the main area of dispute about this process concerns Hitler’s precise role in ordering the killing of the Jews. Remarkably, we simply do not know, in an unequivocal way, what Hitler’s precise role was. The dictator often gave oral orders to senior henchmen such as Heinrich Himmler that were never written down, and historians can only infer Hitler’s precise role and intentions from evidence which is infuriatingly inadequate and contradictory. Because of the lack of unambiguous evidence, historians have been divided for decades, rather misleadingly, into so-called “intentionalists,” who argue that Hitler always intended to kill the Jews, and “functionalists,” who claim that the killing process somehow, as it were, welled up from local SS units in Russia until it became general Nazi policy.
Germany invaded the Soviet Union in June 1941.[236] German propaganda portrayed the war against the Soviet Union as both an ideological war between German National Socialism and Jewish Bolshevism and a racial war between the Germans and the Jewish, Romani and Slavic Untermenschen ("sub-humans").[237] Local populations in some occupied Soviet territories actively participated in the killing of Jews and others, and helped identify and round up Jews.[238] German involvement ranged from active instigation and involvement to general guidance.[239] In Lithuania, Latvia, and western Ukraine, locals were deeply involved in the murder of Jews from the beginning of the German occupation. Some of these Latvian and Lithuanian units also participated in the murder of Jews in Belarus. In the south, Ukrainians killed about 24,000 Jews and some went to Poland to serve as concentration and death-camp guards.[238] Military units from some countries allied to Germany also killed Jews. Romanian units were given orders to exterminate and wipe out Jews in areas they controlled.[240] Ustaše militia in Croatia persecuted and murdered Jews, among others.[168] Many of the killings were carried out in public, a change from previous practice.[241]

One of the most atrocious eras in human history is without a doubt the Holocaust. About 11 million people, including approximately 6 million Jews, are estimated to have been slaughtered at the hands of Adolf Hitler and the Nazi Party. Jews were forced to live in ghettos and then rounded up to be sent to concentration and extermination camps, where they were herded into gas chambers and killed. At a number of concentration camps, Nazi doctors conducted gruesome and horrific medical experiments on prisoners against their will. This leads us to one of the most infamous Nazi doctors who ever lived, Dr. Josef Mengele.

Jews would be “utilised for work in the East… [The] sexes [will be] separated. Jews capable of work will be moved into these areas as they build roads, during which a large proportion will no doubt drop out through natural reduction. The remnant that eventually remains will require suitable treatment…The evacuated Jews will first be taken, group by group, to so-called transit ghettos, in order to be transported further east from there.”
Mengele joined the Nazi Party in 1937 and the Schutzstaffel (SS; protection squadron) in 1938. He received basic training in 1938 with the Gebirgsjäger (light infantry mountain troop) and was called up for service in the Wehrmacht (Nazi armed forces) in June 1940, some months after the outbreak of World War II. He soon volunteered for medical service in the Waffen-SS, the combat arm of the SS, where he served with the rank of SS-Untersturmführer (second lieutenant) in a medical reserve battalion until November 1940. He was next assigned to the SS-Rasse- und Siedlungshauptamt (SS Race and Settlement Main Office) in Poznań, evaluating candidates for Germanization.[18][19]