With numerous new documents that were never published before, this comprehensive study highlights the widely forgotten and neglected twin-camp character of the former Mauthausen Concentration Camp complex with key installations in St. Georgen and Gusen. It supplements the conventional narrative of the history of Concentration Camp Mauthausen with new and previously unpublished information from local sources, survivors, liberators, and archives all around the world. It focuses especially on SS infrastructure at St. Georgen and Gusen that is underrepresented in the history of Concentration Camp Mauthausen and the Holocaust in general. The study also supplements the history of World War II and the SS s strategic involvement in German war production during the final phase of the war with new and previously unpublished information. Thus the study gives special emphasize on joint-venture projects of the Waffen-SS with Steyr-Daimler-Puch AG and Messerschmitt GmbH Regensburg that resulted in the realization of a top secret and most modern underground plant for the serial production of Me-262 jet planes in St. Georgen that might have influenced the course of World War II.