[1][2][3] The victims of death camps were primarily murdered by gassing, either in permanent installations constructed for this specific purpose, or by means of gas vans.

The idea of mass extermination with the use of stationary facilities, to which the victims were taken by train, was the result of earlier Nazi experimentation with chemically manufactured poison gas during the secretive Aktion T4 euthanasia programme against hospital patients with mental and physical disabilities.

[12][13] In 1941, the experience gained in the secretive killing of these hospital patients led to the creation of extermination camps for the implementation of the Final Solution.

By then, the Jews were already confined to new ghettos and interned in Nazi concentration camps along with other targeted groups, including Roma, and the Soviet POWs.

The Nazi's so-called "Final Solution of the Jewish Question", based on the systematic murder of Europe's Jews by gassing, began during Operation Reinhard,[14] after the June 1941 onset of the Nazi–Soviet war.

The adoption of the gassing technology by Nazi Germany was preceded by a wave of hands-on killings carried out by the SS Einsatzgruppen,[15] who followed the Wehrmacht army during Operation Barbarossa on the Eastern Front.

[22] On 13 October 1941, the SS and Police Leader Odilo Globocnik stationed in Lublin received an oral order from Reichsführer-SS Heinrich Himmler – anticipating the fall of Moscow – to start immediate construction work on the killing centre at Bełżec in the General Government territory of occupied Poland.

Notably, the order preceded the Wannsee Conference by three months,[23] but the gassings at Chełmno north of Łódź using gas vans began already in December, under Sturmbannführer Herbert Lange.

[31] The Reinhard extermination camps were under Globocnik's direct command; each of them was run by 20 to 35 men from the SS-Totenkopfverbände branch of the Schutzstaffel, augmented by about one hundred Trawnikis – auxiliaries mostly from Soviet Ukraine, and up to one thousand Sonderkommando slave labourers each.

[32] The Jewish men, women and children were delivered from the ghettos for "special treatment" in an atmosphere of terror by uniformed police battalions from both Orpo and Schupo.

[34] An SS anatomist, Johann Kremer, after witnessing the gassing of victims at Birkenau, wrote in his diary on 2 September 1942: "Dante's Inferno seems to me almost a comedy compared to this.

"[35] The distinction was evident during the Nuremberg trials, when Dieter Wisliceny (a deputy to Adolf Eichmann) was asked to name the extermination camps, and he identified Auschwitz and Majdanek as such.

[38] Prisoners represented about a quarter of the total workforce of the Reich, with mortality rates exceeding 75 percent due to starvation, disease, exhaustion, executions, and physical brutality.

[14] Unlike at Auschwitz, where cyanide-based Zyklon B was used to exterminate trainloads of prisoners under the guise of "relocation", the camps at Treblinka, Bełżec, and Sobibór, built during Operation Reinhard (October 1941 – November 1943), used lethal exhaust fumes produced by large internal combustion engines.

These camps had almost identical design: they were several hundred metres in length and width, and were equipped with only minimal staff housing and support installations not meant for the victims crammed into the railway transports.

[44][45] The Nazis deceived the victims upon their arrival, telling them that they were at a temporary transit stop, and would soon continue to German Arbeitslagers (work camps) farther to the east.

[46] Selected able-bodied prisoners delivered to the death camps were not immediately killed, but instead were pressed into labor units called Sonderkommandos to help with the extermination process by removing corpses from the gas chambers and burning them.

At the camps of Operation Reinhard, including Bełżec, Sobibór, and Treblinka, trainloads of prisoners were murdered immediately after arrival in gas chambers designed exclusively for that purpose.

[14] The mass killing facilities were developed at about the same time inside the Auschwitz II-Birkenau subcamp of a forced labour complex,[47] and at the Majdanek concentration camp.

Auschwitz, Majdanek, and Jasenovac were retrofitted with Zyklon B gas chambers and crematoria buildings as the time went on, remaining operational until war's end in 1945.

[51] SS Obersturmführer Kurt Gerstein of the Institute for Hygiene of the Waffen-SS, told a Swedish diplomat during the war, about life in a death camp.

He recounted that on 19 August 1942, he arrived at Bełżec extermination camp (which was equipped with carbon monoxide gas chambers) and was shown the unloading of 45 train cars filled with 6,700 Jews, many already dead.

The Sonderkommando talked to the victims about life in the camp to pacify the suspicious ones, and hurried them inside; to that effect, they also assisted the aged and the very young in undressing.

Camp Commandant Höss reported that the "men of the Special Detachment were particularly on the look-out for this", and encouraged the women to take their children into the "shower room".

[55] Yet, not every prisoner was deceived by such tactics; Commandant Höss spoke of Jews "who either guessed, or knew, what awaited them, nevertheless ... [they] found the courage to joke with the children, to encourage them, despite the mortal terror visible in their own eyes".

[58] The corpses were additionally found half-squatting, their skin discolored pink with red and green spots, with some foaming at the mouth or bleeding from their ears, exacerbated by the crowding in gas chambers.

As the Auschwitz Camp Commandant Rudolf Höss justified the extermination by explaining the need for "the iron determination with which we must carry out Hitler's orders".

[63] The estimated total number of people who were murdered in the six Nazi extermination camps is 2.7 million, according to the United States Holocaust Memorial Museum.

Some extermination camps that remained uncleared of evidence were liberated by Soviet troops, who followed different standards of documentation and openness than the Western allies did.

In the early 1990s, the Jewish Holocaust organisations debated with the Polish Catholic groups about "What religious symbols of martyrdom are appropriate as memorials in a Nazi death camp such as Auschwitz?"