BAGHDAD (Reuters) - Guerrillas killed three U.S. soldiers and an Iraqi guard in a mortar attack on National Guard headquarters in Samarra, north of Baghdad, Thursday. The latest violence erupted a day after Iraq's interim government announced a new security law giving itself tougher powers to combat a bloody insurgency wracking the country. Eighteen U.S. soldiers and three Iraqi guards were also wounded when guerrillas fired four mortar rounds at the Iraqi National Guard headquarters in Samarra, severely damaging the building, also used by U.S. troops, a military spokesman said. The 1st Infantry Division spokesman said U.S. forces used radar...