4 Americans killed in suicide bombing at Afghan military base

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Four Americans were killed early Saturday morning, in an attack at Bagram Airfield in Afghanistan, said NATO.

U.S. Defense Secretary Ash Carter released a statement, "An apparent suicide bomber has taken the lives of two U.S. servicemembers and two U.S. contractors working on the base."

The attacker wore a suicide vest, and detonated the explosive inside the facility.

The blast also injured 16 American servicemembers and 1 Polish soldier

The Taliban has claimed responsibility for the attack. Zabihullah Mujahid, a spokesman for the insurgent group, said the attack had been planned for four months.

He said the bomber targeted a sports ground where more than a hundred people are training.

The Taliban regularly fire rockets at Bagram from outside its perimeter.

Military officials are now looking into how the suicide bomber infiltrated the base.

The attacker may have been someone who worked there, and had access.

Bagram is located more than 30 miles north of the Afghan capital, Kabul.