In their quest, German scientists happened upon a pesticide used in California in the 1880's, Zyklon B. Inert crystals, when heated, would create a gas that would kill anything.
The delivery method designed involved a soldier in a gas mask on the roof, dropping the crystals through an opening into the "bath house" where 1000 naked people had been shoved. Within 20 agonizing minutes all the screams had stopped, and all 1000 were dead.