Madness (1969)

The action takes place in a nameless country occupied by the German army at the time of World War II. In a small village there's an asylum. A squad of German soldiers arrives to take the insane people to a "walk in the forest". Just before they are ready to go to forest to kill the insane people, a Gestapo officer arrives and from his conversation with the doctor we can find out that there's an enemy agent hiding himself in the asylum. Officer Windisch must find the agent from 583 people and on his searches he is slowly starting to feel himself crazy too.