His 2011 book "Völkermord als Staatsgeheimnis" (Genocide as State Secret) with a substantive preface by Karl Doehring, Director of the Max Planck Institute for International Law in Heidelberg, explored the issue of who knew what when about the Holocaust. He is the first historian to have reviewed the issue in the light of published and unpublished Nuremberg documents, and in the light of interviews with Nuremberg prosecutors and defense attorneys, Holocaust survivors as well as German military judges and politicians. He argued that the policy of exterminating the Jews was "geheime Reichssache", and treated pursuant to Hitler's Order Nr. 1 (Führerbefehl Nr. 1) as a "state secret". Accordingly, although there were diffuse rumours about killings, no one except a very limited number of persons knew exactly what was going on, neither the industrialization of the killing nor the number of victims.