From 1943, the purpose of the camp was switched from quarrying to armaments production in vast underground factories, to protect the industrial output from Allied bombing. Work on the tunnels was begun by the Kellerbau Kommando at the original Gusen camp, which had a high mortality rate. The tunnels at Gusen were initially used for the production of Messerschmitt Bf 109 fighter aircraft. The work took on new urgency after the bombing of the Messerschmitt plant in Regensburg on 17 August 1943. Afterwards, 35 per cent of fighter production derived from Gusen and Flossenbürg. By July 1944, 4,000 Gusen prisoners were working on aircraft production, and 77 trainloads of aircraft parts were exported each month. Other prisoners produced rifles, machine guns, and airplane motors for Steyr-Daimler-Puch AG in 16 large warehouses northeast of the original Gusen camp.