He was tried for war crimes for his involvement in the Holocaust and his knowledge of medical experimentation on concentration camp prisoners, but he was ultimately acquitted and released.

[3] The previous year he had almost fallen foul of the Nazis when he campaigned against Hermann Göring's law banning testing on animals, something Hörlein considered essential to his research.

[5] Hörlein sat on the vorstand of the Deutsche Gesellschaft für Schädlingsbekämpfung, an IG Farben subsidiary company largely concerned with the production of Zyklon B.

[6] He regularly received detailed reports of the human experimentation carried out by Helmuth Vetter, who purchased most of his subjects from the concentration camps.

[7] Towards the end of the war Hörlein, sensing that a German defeat was imminent, moved his operations to Leverkusen and began to maintain a much lower profile.