When the first war crimes investigators entered Mauthausen on May 6, 1945, concentration camps Auschwitz, Majdanek, Bergen-Belsen, and Buchenwald had already fallen into Allied hands. Yet despite the horrific scenes that their Russian, British, and American counterparts had reported from these camps, investigators were scarcely prepared for what they discovered. Aside from the masses of dead that littered the camp, the gas chamber and crematoria provided evidence of mass murder on a near-industrial scale. In the weeks that followed, investigators set to work piecing together the crimes committed within the camp’s walls, deeply affected by their visceral confrontation with atrocity.