A Gestapo report to Himmler ( 17 February 1942 ) on increased Partisan activities stated that `` Increased activity of the bands is chiefly due to atrocities carried out by Ustasha units in Croatia against the Orthodox population . A report to Gestapo chief Heinrich Himmler dated 17 February 1942 on increased Partisan activities stated that `` Increased activity of the bands is chiefly due to atrocities carried out by UstaÅ e units in Croatia against the Orthodox population . ''