The Taliban have claimed responsibility for an attack that killed three American civilian contractors Thursday in Afghanistan.

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On Twitter, a Taliban spokesman said the attacker had infiltrated the Afghan security forces and was working at Kabul's airport. The spokesman said the attacker had “opened fire on invaders" and was killed, the Washington Post reported.

It was not clear if he was the Afghan national who had been named as one of the dead.

The White House on Thursday characterized the Taliban as an “armed insurgency” rather than a terrorist group. Deputy press secretary Eric Schultz made that distinction based on the fact that, though the Taliban make use of “tactics that are akin to terrorism,” they are not interested in operating outside of Afghanistan.

Attacks by Afghan forces on westerners is becoming increasingly common in recent years. The uptick caused the Pentagon and NATO to increase the restrictions on how Western forces interacted with Afghan soldiers in 2012.

In 2011, an Afghan officer shot and killed nine Americans at Kabul's airport. The Taliban claimed responsibility for that attack as well.