KABUL, Afghanistan - An explosion tore through the office of an American defense contractor in the heart of the Afghan capital Sunday, killing as many as six people and seriously injuring several more, officials and residents said. The victims included several apparent U.S. citizens. The blast hit the office of Dyncorp Inc., an American firm that provides security for Afghan President Hamid Karzai on behalf of the United States and works for the U.S. government in Iraq, said Nick Downie of the Afghanistan NGO Security Office. Downie said he and others at the scene pulled several seriously injured people —...