The U.S. Embassy in Kabul says three Americans were killed Thursday in an attack at a hospital in the Afghan capital.

Afghan police say a security guard opened fire at western Kabul’s Cure Hospital, which is run by a U.S.-based charity group. At least one of the dead was a doctor.

Interior Ministry spokesman Sediq Sediqqi said the security guard is now in police custody and an investigation is under way.

“This was an unfortunate incident this morning,” he told reporters. “The attacker was a police security guard there and he opens fire on foreign nationals who went there and unfortunately three of them have been killed. One is injured and the injured has been taken to the hospital. And the police has arrested the attacker as well so we will investigate to find out the motives behind this attack.”

It is unclear whether the Taliban is responsible for the shooting.

Foreign civilians in Afghanistan have been the target of a rising number of attacks this year.

Earlier this month, a police officer shot two Associated Press journalists in the eastern part of the country.

In March, a Swedish journalist was killed outside a Kabul restaurant.

Twenty-one people were killed in January in a bombing at a Lebanese restaurant in Kabul’s diplomatic district.

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