Höss experimented with various gassing methods. According to Eichmann's trial testimony in 1961, Höss told him that he used cotton filters soaked in sulfuric acid for early killings. Höss later introduced hydrogen cyanide (prussic acid), produced from the pesticide Zyklon B, to the process of extermination, after his deputy Karl Fritzsch had tested it on a group of Russian prisoners in 1941. With Zyklon B, he said that it took 3–15 minutes for the victims to die and that "we knew when the people were dead because they stopped screaming."