Erected in 1933, Dachau was the first Nazi concentration camp. Used mainly to incarcerate German political prisoners until late 1938, whereupon large numbers of Jews, Gypsies, Jehovah's Witnesses, homosexuals, and other supposed enemies of the state and anti-social elements were sent as well. Nazi doctors and scientists used many prisoners at Dachau as guinea pigs for experiments. During the war, construction began on a gas chamber, but it never became operational. Dachau was liberated by American troops in April 1945.