An Afghan security guard opened fire on a group of foreign doctors at a Kabul hospital on Thursday morning, killing three American physicians and wounding a U.S. nurse, officials said.

The shooting at Cure International Hospital in western Kabul was the latest in a string of deadly attacks on foreign civilians in the Afghan capital this year.

Two of the dead Americans were a father and son, Minister of Health Soraya Dalil said, adding that the third American was a Cure International doctor who had worked in Kabul for seven years.

Dalil said an American nurse was also wounded in the attack.

“A child specialist doctor who was working in this hospital for the last seven years for the people of Afghanistan was killed and also two others who were here to meet him, and they were also American nationals,” Dalil said. “The two visitors were father and son, and a woman who was also in the visiting group was wounded.”

The attacker was a member of the Afghan Public Protection Force assigned to guard the hospital, according to District Police Chief Hafiz Khan. He said the man’s motive was not yet clear.

The gunman, who was detained, was wounded during the attack and underwent surgery at midday in the same medical facility under heavy police guard, according to Kanishka Bektash Torkystani, a Ministry of Health spokesman.

Later in the afternoon, Dalil, the health minister, said he was recovering from the surgery before being questioned. Initial reports indicate he was shot by other security forces, said Ministry of Interior spokesman Sediq Sediqqi.

“Five doctors had entered the compound of the hospital and were walking toward the building when the guard opened fire on them,” Torkystani said. “Three foreign doctors were killed.”

The U.S. Embassy in Kabul confirmed that three American citizens had been killed in the hospital attack but said it had no other information. It did not confirm Afghan reports that the three were doctors.

According to its website, the Cure International Hospital was founded in 2005 by invitation of the Afghan Ministry of Health. It sees 37,000 patients a year, specializing in child and maternity health as well as general surgery. It is affiliated with the Christian charity Cure International, which operates in 29 countries with the motto “curing the sick and proclaiming the kingdom of God.”

The Afghan capital has seen a spate of attacks on foreign civilians in 2014, a worrying new trend as the U.S.-led military coalition prepares to withdraw most troops by the end of the year.

It was unclear whether the Taliban were behind Thursday’s shooting, though the insurgents have claimed several major attacks that killed foreign civilians this year, an escalation of such attacks after years of mostly targeting foreign military personnel and Afghan security forces.

In January, a Taliban attack on a popular Kabul restaurant with suicide bombers and gunmen killed more than a dozen people, while in March gunmen slipped past security at an upscale hotel in the Afghan capital and killed several diners in its restaurant. Two foreign journalists were killed and another wounded in two separate attacks.

The hospital shooting is also the second “insider attack” by a member of Afghan security forces targeting foreign civilians this month.