Between 1941 and 1943 Tilly was based in Nancy as a "regional TA instructor" for Meurthe-et-Moselle. Later she transferred to Lille to undertake similar duties in respect of the Nord department and Pas-de-Calais. Both these areas were in what then Germans had defined as Zone interdite, subjected to additional movement restrictions on local populations and higher levels of Gestapo and military supervision than most of occupied northern France. Sources are understandably vague about the details of Spiegel's resistance work, but there can be little doubt that as a communist resistance member easily identifiable as a non-local, her activities would placed her in extreme danger. In 1943 the focus of her activities evidently returned to Paris: it was probably in August 1944 that she was arrested by the Gestapo and detained, like her husband, in Fresnes Prison, a short distance to the south of the city. They were scheduled for execution. By the end of the month Paris had been liberated and both were released in the turmoil that accompanied the German military withdrawal.