Four U.S. NATO troops were killed when an Afghan police officer turned his gun on them at a routine checkpoint early Sunday morning in what's being called an "insider" attack, but no one's sure why. 

Officials are looking for a reason why a member of the Afghan police turned on allied forces Sunday morning and engaged a NATO team in a fire fight. By the time a nearby response team arrived, four American NATO troops were dead and two others were wounded. One of six Afghan police officers assigned to man the post was also dead, while the others had run away. Officials are wondering if the attack was a response to a NATO airstrike that angered locals on Saturday, or if it was something else. The same checkpoint was attacked by insurgents around midnight that night, and some are thinking there might be a connection between the two events. 

Per the Guardian:

Afghan officials said the checkpoint in Zabul province's Mizan district came under attack first from insurgents sometime around midnight. American forces came to help the Afghan police respond to the attack, said Ghulam Gilani, the deputy police chief of the province.

Gilani said he wasn't sure if the two events were related or not. The police officers might have been coaxed into attacking the NATO troops. "The checkpoint was attacked last night. Then the police started fighting with the Americans. Whether they attacked the Americans willingly we don't know," Gilani said.

Reuters points out that 'insider' attacks are becoming increasingly more common. Two British NATO soldiers were killed on Saturday by two men dressed as Afghan police officers, and Friday's attack at Camp Bastion that destroyed six fighter jets was carried out by insurgent dressed in U.S. Army uniforms. 

Want to add to this story? Let us know in comments or send an email to the author at connorbsimpson at gmail dot com. You can share ideas for stories on the Open Wire.