On 10 August 1943, he was arrested with his comrades by the Gestapo and sent to Moabit Prison in Berlin. He sat in a cell with Belgian patriot and resistance fighter André Timmermans and a Polish prisoner. Cälil studied the German language in prison to communicate with his cellmates. In prison, he compiled verses composed in the prison into self-made notebooks. He and his group of 12 were sentenced to death on 12 February 1944 and guillotined at Plötzensee Prison, Berlin, on August 25 . His body was never recovered.