Letter from al Qaeda in U.S. militant found after he was slain , Iraq says . Letter urges al Qaeda in Iraq to fight against Sunni -backed American militias . Document also cautions vigilance against infiltrators .
Letter from al Qaeda in Iraq militant found after he was slain , U.S. says . Letter urges al Qaeda in Iraq to fight against American-backed Sunni militias . Document also cautions vigilance against infiltrators .