That same year, the Gestapo intercepted communications revealing the existence of the Red Orchestra and leading to numerous arrests. Many of those arrested were later executed, including Harnack's brother on 22 December 1942, and on 16 February 1943 his sister-in-law, Mildred Harnack, an American citizen. During this period, Ramdohr was engaged to Falk Harnack, which Arvid mentioned in his farewell letter to his family, written hours before his execution.[5]