Höss experimented with different poison gases. At first, he used cotton filters soaked in sulfuric acid. Höss later started using hydrogen cyanide (prussic acid), made from the pesticide Zyklon B, after his deputy Karl Fritzsch tested it on a group of Russian prisoners in 1941. Höss said that when he used Zyklon B, it took 3–15 minutes for prisoners to die, and that "we knew when the people were dead because they stopped screaming."