Auschwitz-Birkenau was a complex, consisting of a concentration camp, a forced labour camp and an extermination camp. Eventually it had a network of more than 40 satellite camps. Following tests in September 1941, the lethal gas Zyklon B was selected as the method of murder. Auschwitz initially had one gas chamber at the Auschwitz I camp, but this was soon expanded. By 1943, four new crematoria, with gas chambers attached, had been built in Auschwitz II. Approximately 1.1 million people were murdered in the Auschwitz gas chambers.