As the world marks the 75th anniversary of the liberation of Auschwitz, we speak to historian Christoph Kreutzmüller, who has been studying the photographs taken by SS officers at the Nazi death camp. He tells us why these pictures are such an important source of evidence, despite being taken by the perpetrators. If they had been examined earlier, at least one suspect who was acquitted in court would have been found guilty, Kreutzmüller explains. He joined us for Perspective.

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