In the early 20th century, this became a centre of the engineering and arms industries. During World War II, the Nazi German government established and operated the nearby KZ Mittelbau-Dora concentration camp, where 60,000 forced labourers had to work in the arms industry. They were prisoners of war and persons from occupied territories. Some 20,000 persons died because of the bad conditions. In April 1945, most of the city was destroyed by Royal Air Force bombings, resulting in 8,800 casualties (more than 20% of the population). Most of the historic buildings in the city were destroyed; it suffered the most damage during the war of any city in Thuringia. A week later the United States troops occupied the city, followed weeks later by the Soviet Red Army. The city was within the Soviet zone of occupation, and later the territory was known as East Germany. Hundreds of German scientists and their families from Nordhausen were among thousands deported to the Soviet Union after the war to work on advanced rocket and other arms engineering projects.