Later the old gaol became the Brandenburg Euthanasia Centre where the Nazis killed people with mental problems, including children. They called this operation "Action T4" because of the Berlin address, Tiergartenstraße 4, the headquarters of this planned and well-organized killing "euthanasia" organisation. Brandenburg an der Havel was one of the very first locations in the Third Reich where the Nazis experimented with killing their victims by gas.