4 Americans killed in blast at air base in Afghanistan

The U.S. military says four Americans have been killed in an explosion at the Bagram air field in Afghanistan.
Defense Secretary Ash Carter says two U.S. servicemen and two American contractors were killed. Another 16 U.S. service members and one Polish soldier were wounded.
The Taliban claimed responsibility for the attack, which they said was carried out by a suicide bomber inside the base. The Taliban regularly fire rockets at the base north of Kabul from outside its perimeter.
A spokesman fo