Here is part of Auschwitz Commandant Rudolf Hoess’s official testimony at the Nuremburg Trials, 1946:
The “final solution” of the Jewish question meant the complete extermination of all Jews in Europe. l was ordered to establish extermination facilities at Auschwitz in June 1941. At that time there were already in the general govemment three other extermination camps; BELZEK, TREBLINKA and WOLZEK. These camps were under the Einsatzkommando of the Security Police and SD. I visited Treblinka to find out how they carried out their exterminations. The Camp Commandant at Treblinka told me that he had liquidated 80,000 in the course of onehalf year. He was principally concerned with liquidating all the Jews from the Warsaw Ghetto. He used monoxide gas and I did not think that his methods were very efficient. So when I set up the extermination building at Auschwitz, l used Cyclon B, which was a crystallized Prussic Acid which we dropped into the death chamber from a small opening. It took from 3 to 15 minutes to kill the people in the death chamber depending upon climatic conditions. We knew when the people were dead because their screaming stopped. We usually waited about onehalf hour before we opened the doors and removed the bodies. After the bodies were removed our special commandos took off the rings and extracted the gold from the teeth of the corpses.
Source: Modern History Sourcebook (an internet project of Fordham University). More of Hoess’s testimony here.