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KABUL, Afghanistan (AP) – A police officer opened fire on U.S. and Afghan forces inside a police headquarters in eastern Afghanistan on Monday, sparking a firefight that killed two U.S. troops and two other Afghan policemen. The attacker was also killed in the shootout, officials said.

The incident in Wardak province was the latest in a series of insider attacks against coalition and Afghan forces that have threatened to undermine their alliance at a time when they need to work increasingly close together in order to hand over responsibility as planned next year.

In Monday’s attack, an Afghan police officer stood up in the back of a police pickup truck, grabbed hold of a machine gun and started firing at the U.S. special operations forces and Afghan police in the police compound in Jalrez district, said the province’s Deputy Police Chief Abdul Razaq Koraishi.

The assailant killed two Afghan policemen and wounded four, including the district police chief, before he was gunned down, Koraishi said. He did not have a death toll for the U.S. troops.