In January 1942 he undertook to deliver by hand a letter from the Conference of Dutch Bishops to the editors of Catholic newspapers in which the bishops ordered them not to print official Nazi documents, as was required under a new law by the German occupiers. He had visited fourteen editors before being arrested on 19 January at the Boxmeer monastery.
After being held prisoner in Scheveningen, Amersfoort, and Cleves, Brandsma was transferred to the Dachau concentration camp, arriving there on 19 June. His health quickly gave way, and he was transferred to the camp hospital. He died on 26 July 1942, from a lethal injection administered by a nurse of the Allgemeine SS, as part of their program of medical experimentation on the prisoners.