An Afghan woman wearing a police uniform shot dead on Monday a civilian contractor working for Western forces in the police chief's compound in Kabul, NATO said.
The incident is likely to raise troubling questions about the direction of an unpopular war.
It appeared to be the first time that a woman member of Afghanistan's security forces carried out such an attack.
There were conflicting reports about the victim.
A spokesman for the NATO-led International Security Assistance Force (ISAF) said a US police adviser was killed by an Afghan policewoman. Then ISAF said in a statement only that it was a "contracted civilian employee" who was killed.
Mohammad Zahir, head of the police criminal investigation department, described the incident as an "insider attack" in which Afghan forces turn their weapons on Western troops they are supposed to be working with. He initially said the victim was a US soldier.
After more than 10 years of war, militants are capable of striking Western targets in the heart of the capital, and foreign forces worry that Afghan police and military forces they are supposed to work with can suddenly turn on them.
The policewoman approached her victim as he was walking in the heavily guarded police chief's compound in a bustling area of Kabul. She then drew a pistol and shot him once, a senior police official told Reuters.
The police complex is close to the Interior Ministry where in February, two American officers were shot dead at close range at a time anger gripped the country over the burning of copies of the Quran at a NATO base.
"She is now under interrogation. She is crying and saying 'what have I done'," said the official, of the police officer who worked in a section of the Interior Ministry responsible for gender awareness issues.
Tips for troops
The insider incidents, also known as green-on-blue attacks, have undermined trust between coalition and Afghan forces who are under mounting pressure to contain the Taliban insurgency before most NATO combat troops withdraw by the end of 2014.
Security responsibilities in a country plagued by conflict for decades will be handed to Afghan security forces.
Many Afghans fear a civil war like one dominated by warlords after the withdrawal of Soviet occupying forces in 1989 could erupt again, or the Taliban will make another push to seize power if they reject a nascent peace process.
At least 52 members of the NATO-led International Security Assistance Force have been killed this year by Afghans wearing police or army uniforms.
Insider attacks now account for one in every five combat deaths suffered by NATO-led forces in Afghanistan, and 16 percent of all US combat casualties, according to 2012 data.
Hoping to stop the increase in the attacks, Afghan Defense Ministry officials have given their troops tips in foreign culture.
They are told not to be offended by a hearty pat on the back or an American soldier asking after your wife's health.
NATO attributes only about a quarter of the attacks to the Taliban, saying the rest are caused by personal grievances and misunderstandings. Last year, there were 35 deaths in such attacks.