Höss began testing and perfecting mass murder techniques on 3 September 1941. His experiments made Auschwitz the camp that would kill the most prisoners during the Holocaust. Höss later said that, during a regular day at Auschwitz, two to three trains carrying 2,000 prisoners each would arrive every day for periods of four to six weeks. The prisoners were unloaded in the Birkenau death camp. Prisoners who were strong, healthy, and able to work were marched to barracks in either Birkenau or one of the other Auschwitz camps. Prisoners who were elderly, very young, sick, or unable to do forced labor were killed in gas chambers, which were disguised as showers. At first, small gassing bunkers were located deep in the woods, to keep them secret. Later, four large gas chambers and crematoria were built in Birkenau to make it easier for the Nazis to kill more people, more quickly.