Rudolf Hoess found that gassing by carbon monoxide, the recommended method, was inefficient and introduced the cyanide gas Zyklon B. He later recalled: "The gassing was carried out in the detention cells of Block 11. Proctected by a gas mask, I watched the killing myself. In the crowded cells, death came instantaneously the moment the Zyklon B was thrown in. A short, almost smothered cry, and it was all over... I must even admit that this gassing set my mind at rest, for the mass extermination of the Jews was to start soon, and at that time neither Eichmann nor I was certain as to how these mass killings were to be carried out. It would be by gas, but we did not know which gas and how it was to be used. Now we had the gas, and we had established a procedure."