The Nazis established killing centers for efficient mass murder. Unlike concentration camps, which served primarily as detention and labor centers, killing centers (also referred to as "extermination camps" or "death camps") were almost exclusively "death factories." German SS and police murdered nearly 2,700,000 Jews in the killing centers either by asphyxiation with poison gas or by shooting. The first killing center was Chelmno, which opened in December 1941 in the part of Poland annexed to Germany. Mostly Jews, but also Roma (Gypsies), were gassed in mobile gas vans there. In 1942, the Nazis opened the Belzec, Sobibor, and Treblinka killing centers (known collectively as the Operation Reinhard camps) to systematically murder the Jews of Poland. In the Operation Reinhard killing centers, the SS and their auxiliaries killed approximately 1,526,500 Jews between March 1942 and November 1943.
Almost all of the deportees who arrived at the killing centers were sent immediately to death in the gas chambers. The largest killing center was Auschwitz-Birkenau, which by spring 1943 had four gas chambers (using Zyklon B poison gas) in operation. At the height of the deportations, up to 6,000 Jews were gassed each day at Auschwitz-Birkenau in Poland. Over a million Jews and tens of thousands of Roma, Poles, and Soviet prisoners of war were killed there by November 1944. The SS considered the killing centers top secret. To obliterate all traces of gassing operations, special prisoner units were forced to remove corpses from the gas chambers and cremate them. The grounds of some killing centers were re-landscaped or camouflaged to disguise the murder of millions.