Who built the gas chambers? What kind of gas was used to kill Jews there, and who provided it?

In the extermination camps built under Operation Reinhard – Belzec, Sobibor, and Treblinka – the gas first used to murder people was carbon monoxide, generated by gasoline engines or released from carbon-monoxide cylinders. Experiments in the use of Zyklon B, a form of hydrogen cyanide or prussic acid, began in Auschwitz in September 1941. The gas pellets were supplied by DEGESCH (a German cooperative that manufactured pesticides), which was controlled by I.G. Farben. Tesch and Stabenow Co. of Hamburg also supplied gas pellets. The improved gas-chamber and crematoria facilities at Auschwitz-Birkenau were built by J. A. Topf und Sohne of Erfurt, Germany.