Clashes on Saturday in Karbala left five Americans dead. Lt. Col. Scott R. Bleichwehl, an American military spokesman, said gunmen who stormed the provincial governor’s office during a meeting between American and local officials were wearing American military uniforms in an effort to impersonate U.S. soldiers. It is not uncommon for murderers to impersonate Iraqi security forces, but this may be the first time that attackers disguised themselves as Americans.