Auschwitz I, the original concentration camp and the administrative centre of the camp complex, was set up in former Polish army barracks on the outskirts of the village. Auschwitz I was structured similarly to concentration camps in Germany and initially it was used mainly for imprisoning intellectuals and people from the Polish resistance movement. Later, other prisoners were sent to the camp: Soviet prisoners of war, ordinary criminals, Jews, homosexuals, and others. According to estimates, 70,000 people were murdered in Auschwitz I. Many prisoners died of exhaustion, starvation, contagious diseases, harsh labour conditions, or were murdered by firing squads. Auschwitz prisoners often had to perform forced labour outside of the camp and were marched daily via the entrance gate, which was cynically crowned with the motto, Arbeit macht frei (work makes free). In the autumn of 1941, the first experiments of killing people with gas were carried out in prisoners block II of Auschwitz I. 850 Polish Jews and Soviet prisoners were gassed with a herbicide based on hydrogen cyanide (trade name: Zyklon B). This paved the way for the large-scale use of Zyklon B for extermination in the gas chambers of Auschwitz-Birkenau.