Zyklon B was a stabilized preparation of hydrogen cyanide, originally developed for killing insects and rats, but, with modifications, was the chemical used for genocide in the Auschwitz gas chambers. It was originally patented by I.G. Farben, and licensed to the Tesch/Stabenow and Degesch companies. Officers of those companies were tried by a British postwar court and two were hanged for complicity in crimes against humanity.
The basic preparation, for commercial use, had the hydrogen cyanide adsorbed onto an inert substrate, along with a warning agent that would cause coughing and choking below the lethal level of the toxic agent. When put into open air, especially if warmed, it would release lethal quantities of the poison. For use in genocide, the warning agent was removed so the victims would not sense the growing lethal concentration.