Other conspiracy theories point to the British, the German Abwehr intelligence agency or the Poles themselves, some of whom (especially under the command of General Anders) had shown animosity toward Sikorski for, at least from their point of view, his policy of "colluding" with Soviet Premier Joseph Stalin, which reached new heights by the Germans' April 1943 discovery of mass graves filled with thousands of Polish prisoners of war murdered by the Soviets at Katyn Forest in 1940. In 1967 the Holocaust denier David Irving published a book dedicated to this incident, and suggesting British complicity in the incident. Irving's method and selective use of sources have been the subject of significant criticism.