https://en.wikipedia.org/wiki/Zyklon_B
Hydrogen cyanide, a poisonous gas that interferes with cellular respiration, was first used as a pesticide in California in the 1880s.
Research at Degesch of Germany led to the development of Zyklon (later known as Zyklon A), a pesticide that released hydrogen cyanide upon exposure to water and heat. It was banned after World War I, because some of its components were used in German chemical weapons.[2]