KABUL, Afghanistan — An Afghan police officer killed at least 10 of his fellow officers on Saturday, a day after six U.S. service members were gunned down by their Afghan partners in summer violence that has both international and Afghan forces questioning who is friend or foe. ... A day earlier, two Afghans shot and killed six American service members Friday in neighboring Helmand province in the south where insurgents have wielded their greatest influence. In the first attack, an Afghan police officer shot and killed three Marines after sharing a pre-dawn meal with them in the volatile Sangin district,...