Dachau was the first Nazi concentration camp, opening in 1933 and serving as a model for other camps. It imprisoned Jews, homosexuals, immigrants, Jehovah's Witnesses and others. In 1941, Hitler and Himmler formed the Einsatzgruppen to find and kill Jews or bring them to camps. Prisoners at Dachau were subjected to torture and medical experiments by camp guards, including cutting women's breasts and beating prisoners with barbed wire. The camp was liberated by American troops on April 29, 1945, resulting in many guards being killed by freed prisoners.