Experiences with the mass murder of people were shared by almost all of the approximately 120 Germans and Austrians who were deployed between late 1941 and late 1943 as part of the Aktion Reinhardt in the three extermination camps of Belzec, Sobibor, and Treblinka. They had acquired their knowledge previously in the euthanasia facilities and the Berlin T4 central office, which they now used to design the camps and gas chambers. They acted as supervisors, coordinated the guard teams, and several did not limit themselves to killing on command but also abused and killed out of sheer caprice. With the systematic murder of European Jews as part of Aktion Reinhardt, the men from T4 ultimately became experts in extermination. Sara Berger impressively depicts the close network of relationships, analyzing obedience and group pressure.