Secretary of Defense Leon Panetta is in Afghanistan on Wednesday. As he arrived, a vehicle exploded at the base where the secretary was scheduled to speak. Panetta is unhurt, and it's not clear whether it was an attack on the secretary or not. His long-planned trip comes in the middle of a crisis in relations between the U.S. and Afghanistan. The U.S. is investigating the killings of 16 Afghan civilians, apparently committed by a U.S. soldier.