A massive explosion in Afghanistan Sunday killed six American troops and wounded four more. The soldiers were stationed at a new outpost in a remote area when a van loaded with explosives was detonated on a narrow road just outside the small building. It wasn’t immediately clear how the van managed to get so close. The blast, so large that it could be heard eight miles away, caved in the structure’s walls and buried the soldiers. The Taliban claimed responsibility for the attack: “We have killed numbers of Americans and Afghan soldiers and wrecked and ruined their security check post,” the group said.