The camps used either carbon monoxide or cyanide gas, with the first gassing occurring late 1941 and the last in August 1944. Cyanide was introduced in Auschwitz-Birkenau extermination camp by the use of Zyklon B, an insecticide, which was thought to be more effective. Carbon monoxide took many minutes to kill, even with the chamber crowded with hundreds of people. Also, in Auschwitz-Birkenau the method of introducing the people to the chambers were more efficient, with large underground rooms disguised as shower rooms, into which the victims walked mostly without resistance and calmly left their clothes and belongings. Because of these reasons, Auschwitz-Birkenau was thought of as the most efficient part of the Nazi killing machine during the Holocaust, and could kill up to 2,000 people a day if a proper schedule was made and followed.