The group was on a combined security patrol in the Wardak province in the central region of Afghanistan.
According to a spokesman for the Wardak provincial governor, the troops were killed by "an individual wearing an ANP uniform." After he opened fire, the officer fled the scene.
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The AP reports a joint team of Afghan and American officials were investigating the attack.
"However tragic, this event will not hamper the close partnership and combined security efforts [between international and Afghan security forces]," said Zemarai Bashary, spokesman for the Afghan Interior Ministry.