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Instructions for the Government of Armies of the United States in the Field (Lieber Code). 24 April 1863.
Section V : Safe-conduct -- Spies -- War-traitors -- Captured messengers -- Abuse of the flag of truce - Art. 104.
Art. 104. A successful spy or war-traitor, safely returned to his own army, and afterwards captured as an enemy, is not subject to punishment for his acts as a spy or war-traitor, but he may be held in closer custody as a person individually dangerous.