Three Americans have been shot dead by a policeman at a hospital in the Afghan capital, Kabul, US officials say.
A spokesman for Afghanistan's interior ministry described those killed as "medical staff".
The hospital, which specialises in children's and maternal health, is run by Cure, a small US Christian charity. Two others were reported wounded.
Family members identified one of the dead as Dr Jerry Umanos, who lived in Chicago.
The attacker, who reportedly shot himself, is in police custody. His motive remains unclear, officials say.
Afghanistan suffered a spate of deadly attacks in the run-up to presidential elections on 5 April.
The US embassy in Kabul tweeted that it was "with great sadness we confirm that three Americans were killed in the attack on Cure Hospital". No other information would be released for the moment, it added.
"Our families and friends have suffered a great loss, and our hearts are aching," said Jan Schuitema, Dr Umanos' wife, from outside the family home in Chicago on Thursday.
"We don't hold any ill will towards Afghanistan in general or even the gunman who did this. We don't know what his history is."
The two other victims have not been identified.
According to Kabul police, a group of five or six foreigners were entering the hospital in western Kabul early on Thursday when a policemen at a nearby checkpoint opened fire before turning his weapon on himself.
The policeman was named as Ayunullah.
The BBC's Bilal Sarwary in Kabul says he worked for the Afghan Public Protection Force (APPF), which is assigned to guard the hospital.
The BBC understands Mr Ayunullah's family members are being questioned in the eastern city of Jalalabad.
Cure took over the 100-bed hospital seven years ago and restored it to specialise in providing healthcare for women and children. The hospital employs 27 doctors and 64 nurses, the charity says.
Thursday's is the latest high-profile attack in the Afghan capital in recent months.
There were a spate of deadly attacks in the run-up to presidential elections held on 5 April, including a Kabul restaurant bombing in January and an attack on a hotel in the city in March.
In the same month, the Taliban attacked a guest house used by foreigners working for a US-based agricultural charity, Roots for Peace.
More than seven million Afghans turned out to vote earlier this month, defying threats by Taliban militants to disrupt the poll.
With President Hamid Karzai stepping down after two terms in office, the election should led to the country's first democratic transfer of power.
Votes cast in the first round are still being counted. A run-off will take place in late May if no candidate secures a majority.