The U.S. won't yet confirm that it was an American killed by one of Afghanistan's special forces troops. If true, it would mark an ominous turn.KABUL, Afghanistan — For the first time, a member of Afghanistan's elite special forces has carried out a deadly attack against an American military mentor, a senior Afghan army official said Friday, an ominous escalation in the "green-on-blue" shootings that have threatened Western troops' partnership with the Afghan police and army.