Jan 17, 8:14 PM (ET) NEW YORK (Reuters) - Iraqi guerrillas are using increasingly sophisticated weapons and tactics to attack U.S. aircraft, according to a classified Army study on the downing of helicopters in Iraq, The New York Times reported in Sunday editions. Citing senior Army officials in Iraq and the Gulf who were familiar with the study, the newspaper reported that at least one advanced missile was used by insurgents. Rebels have used rocket-propelled grenades and heat-seeking surface-to-air missiles, the latter which require a degree of skill, in the attacks. Scores of U.S. servicemen have died in recent months...