During the retreat from the Soviet Union, German officers destroyed incriminating documents.[17] Wehrmacht soldiers actively worked together with the Schutzstaffel (SS) paramilitary death squads of Nazi Germany that were responsible for mass killings, the Einsatzgruppen, and participated in the mass killings with them such as at Babi Yar.[18] Wehrmacht officers considered the relationship with the Einsatzgruppen to be very close and almost cordial.[19]