As was revealed only 60 years after the fact, Schlabrendorff wrote analyses for the US secret service OSS about the leadership of the Wehrmacht and about war crimes committed by the Nazis. OSS head William J. Donovan then questioned him personally. When Donovan became adviser to the American delegation of lawyers at the Nuremberg trials after the dissolution of the OSS, he included Schlabrendorff on his staff. Schlabrendorff convinced Donovan that it was not the Germans but the Soviet secret service NKVD that had murdered some 4000 Polish officers in the Katyn forest. Donovan then had the Americans block the Soviet attempt to add Katyn to the list of German war crimes.