Story highlights Three Americans were killed by an Afghan policeman, an official says

A NATO spokesman says the three were contractors, but did not confirm nationalities

An Afghan policeman opened fire at a training center in western Afghanistan on Sunday, killing three Americans, a police official told CNN.

The Afghan official, who declined to be named, said the three victims were most probably trainers at the West Zone Police Training Center in Herat province. The shooter was also killed, the official said.

NATO spokesman Maj. Adam Wojack said the three killed were civilian contractors working for the International Security Assistance Force. He could not confirm their nationality or what their specific jobs were.

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The Afghan police official said it was not immediately known whether the attacker had links to any insurgent groups. The motive for the shooting was also unknown.

More information will be released Monday, he said.