An Afghan policewoman has killed an American adviser at the Kabul police headquarters - another "insider attack" that is bound to raise troubling questions about the direction of an unpopular war.
It appeared to be the first time a woman member of Afghanistan's security forces shot a member of the Western coalition force supporting and training Afghanistan's military and police.
"A US police adviser was killed in an attack by an Afghan policewoman," a spokesman for the NATO-led International Security Assistance Force (ISAF) said.
Mohammad Zahir, head of the police criminal investigation department, described Monday's incident as an "insider attack" in which Afghan forces turn their weapons on Western troops they are supposed to be working with.
After more than 10 years of conflict, militants are capable of staging attacks on Western targets in the heart of the capital, and foreign forces worry that members of the Afghan police and military they are supposed to work with can suddenly turn on them.
The policewoman approached the American adviser as he was walking in the heavily guarded police chief's compound in a bustling area of the capital. She then drew a pistol and shot him once, a senior police official told Reuters.
The police compound 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 Muslim holy book 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.
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 per cent of all US combat casualties, according to 2012 data.
Hoping to stop the alarming rise in the attacks, Afghan Defense Ministry officials have given their troops tips in foreign culture, telling them 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.