The first experimental gassing took place around August 1941, when Lagerführer Karl Fritzsch, at the instruction of Rudolf Höss, killed a group of Soviet prisoners of war by throwing Zyklon B crystals into their basement cell in block 11 of Auschwitz I. A second group of 600 Soviet prisoners of war and around 250 sick Polish prisoners were gassed on 3–5 September. The morgue was later converted to a gas chamber able to hold at least 700–800 people. Zyklon B was dropped into the room through slits in the ceiling.