The gas chamber was recorded to be first used to execute in Nevada, 1924. Later during World War II (1939-1945) the Nazis under order of Adolf Hitler, used this same technology to execute mass numbers of Jews and other minorities held by the German army in concentration camps. There actions were held as War Crimes, and all involved were punished in the Nuremberg trials.
When one is sentenced to death by lethal gas they are then strapped down into a chair located in an airtight chamber, the gas chamber. When the process begins small glass bubbles filled with cyanide are dropped from the bottom of the chair and break into a container of sulfuric acid. Once the two substances mix they form the deadly gas of hydrocyanic acid. Within seconds of the intake of this gas the person loses all conciousness and eventually dies within five minutes.
A total of six states still consider the gas chamber a legal form of execution.
Although most of these states only use consider the gas chamber as a back up to the lethal injection, it is still somewhat active in the state of Arizona. It was most recently used on March 3, 1999, in the death of Walter LeGrand.