The Auschwitz concentration camp complex was the largest of its kind established by the Nazi regime. It included three main camps. All three camps used prisoners for forced labor. The SS authorities established three main camps near the Polish city of Oswiecim: Auschwitz I in April 1940; Auschwitz II (also called Auschwitz-Birkenau) in October 1941; and Auschwitz III (also called Auschwitz-Monowitz) in October 1942. Construction began in April 1940 in an abandoned Polish army barracks for Auschwitz I. Prisoners began adding second stories to single story blocks.