D, Cogdell, Cooley, and Walker, federal prisoners, crawled through a window from which a bar had been removed, slid down a knotted bedsheet, and escaped from custody. They were all recaptured within about 3 months. All were charged with violating 18 U.S.C. § 751(a), which governs escape from federal custody. Each was convicted by the jury. Ds' defense of duress or necessity centered on the conditions in the jail during the months of June, July, and August 1976, and on various threats and beatings directed at them during that period. Construed in the light most favorable to them, this evidence demonstrated that the inmates and on occasion the guards in that unit, set fire to trash, bedding, and other objects thrown from the cells. According to the inmates, the guards simply allowed the fires to burn until they went out. Although the fires apparently were confined to small areas and posed no substantial threat of spreading through the complex, poor ventilation caused smoke to collect and linger in the cellblock. Ds also introduced testimony that the guards at the jail had subjected them to beatings and to threats of death. The District Court stressed that, to sustain their defenses, respondents would have to introduce some evidence that they attempted to surrender or engaged in equivalent conduct once they had freed themselves from the conditions they described. No such evidence was forthcoming. The District Court refused to submit to the jury any instructions on Ds' defense of duress or necessity. The Court of Appeals reversed holding that the District Court had improperly precluded consideration by the respective juries of Ds' tendered evidence of the conditions at the jail. The majority concluded that the District Court should have allowed the jury to consider the evidence of coercive conditions in determining whether the respondents had formulated the requisite intent to sustain a conviction under § 751 (a). The majority in the Court of Appeals assumed that escape, as defined by § 751 (a), was a 'continuing offense' as long as the escapee was at large. Given this assumption, the majority agreed with the District Court that, under normal circumstances, an escapee must present evidence of coercion to justify his continued absence from custody as well as his initial departure. The instructions given to the jury presupposed that Ds were being tried for the initial escape and that the jury was not aware that escape was a continuing offense. This failure, according to the majority, constituted 'an obvious violation of [respondents'] constitutional right to jury trial.' The Supreme Court granted certiorari.