Höss began testing and perfecting techniques of mass murder on 3 September 1941. His experiments led to Auschwitz becoming the most efficiently murderous instrument of the "Final Solution" and the Holocaust's most potent symbol. According to Höss, during standard camp operations, two or three trains carrying 2,000 prisoners each would arrive daily for four to six weeks. The prisoners were unloaded in the Birkenau camp; those fit for labour were marched to barracks in either Birkenau or one of the Auschwitz camps, while those unsuitable for work were driven into the gas chambers. At first, small gassing bunkers were located deep in the woods to avoid detection. Later, four large gas chambers and crematoria were constructed in Birkenau to make the killing process more efficient, and to handle the increasing rate of extermination.