DUBAI (Reuters) - A little-known Iraqi insurgent group said on Tuesday it was holding a U.S. soldier and threatened to kill him within 72 hours if Iraqi prisoners were not released, according to an Internet statement. "Our mujahideen ... have managed to capture the American soldier John Adam after killing a number of his colleagues," said the Mujahideen Squadrons in the undated statement. It carried a picture appearing to show a U.S. soldier sitting in front of a black banner with a rifle pointed at his head. The authenticity of the claim, which did not say where the man was...