Three Americans – a pediatrician and a father and son – were killed by an Afghan government security officer at a hospital Thursday, the latest in a series of attacks on foreign civilians that has rattled aid workers, contractors and journalists.

Another American female medical worker was wounded in the attack at Cure International Hospital of Kabul, run by a U.S.-based Christian charity, and the gunman also was wounded, officials said.

The hospital staff performed surgery on the attacker, who had shot himself before he was handed over to Afghan authorities, Cure said in a statement. However, Interior spokesman Sediq Sediqqi said the assailant was shot by other security guards.

The attacker’s motive was not clear, police said, and there was no Taliban claim of responsibility by Thursday night.

As international troops withdraw, civilian workers increasingly fear they are considered prime targets by militants. Some are rethinking their safety – and even if they will stay.

All three of the dead were identified as American doctors by Bektash Torkystani, a Health Ministry spokesman. But the U.S. Embassy confirmed only that three American citizens had been killed. Cure said a doctor was one of three people killed.

Among the dead was Dr. Jerry Umanos, a 57-year-old pediatrician from Chicago, according to his mother-in-law, Angie Schuitema. The Lawndale Christian Health Center in Chicago said Umanos worked there for more than 16 years before moving to Afghanistan in 2005.

Health Minister Soraya Dalil said the other two dead Americans were a father and son, who were visiting, and a U.S. nurse was wounded. The father and son were reportedly also from Illinois.

The shooting continued a deadly pattern of attacks on civilian targets in Kabul.

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

But attacks on Western civilians have not been limited to Kabul. On April 4, an Afghan police officer shot two Associated Press journalists working in the eastern province of Khost, killing photographer Anja Niedringhaus and wounding veteran correspondent Kathy Gannon.

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

While aid groups have been targeted before, the frequency of such attacks has disturbed a community used to the daily risk of working in conflict zones.

“We’re not seeing aid workers running for the airport, but many organizations are taking a careful look at their security postures,” Graeme Smith, a senior analyst in Kabul for the International Crisis Group, said. “The hard reality is that the country is becoming more violent, and Kabul has not escaped this pattern.”

Violence has spiked overall in Afghanistan as insurgents sought to disrupt the April 5 presidential election and sow insecurity ahead of the troop withdrawal, nearly 13 years after the U.S.-led invasion to topple the Taliban’s radical Islamic regime.

Afghan civilians, of course, have suffered the longest. A U.N. report said 2,959 Afghan civilians were killed last year, up 7 percent. Most of those deaths were caused by the insurgency, many of them by the thousands of roadside bombs planted around the country.

Foreign workers who once moved relatively freely – if carefully – in the capital are taking even more precautions. Instead of shopping at bazaars, traveling in taxis and lunching in cafes, many now are on virtual lockdown, shying away from once-popular restaurants at night. Many aid organizations have long had a system of restricting movements during heightened security risks, but these days that state feels nearly constant.

The increased number of attacks raises the possibility that insurgents have embarked on a campaign against foreign aid workers to drive them away and undermine any help the government might get after most international troops leave at the end of the year.

“Something rather worrying about Taliban attacks this year is that they truly are targeting foreign civilians now,” Kate Clark, head of the Kabul office for the Afghanistan Analysts Network, said. She noted, however, that the Taliban had not claimed responsibility for Thursday’s shooting nor for two other attacks on foreigners this year.

Complicating the picture in the hospital shooting is that it was an “insider attack” by a member of Afghan security forces. Until recently, such attacks mostly targeted foreign military or Afghan forces, and it has for years been difficult to determine whether these were Taliban-influenced or the result of personal disputes.

After so many years of an international presence, many Afghans appear to have shifted views on foreigners in general from celebrating them as liberators to resenting them as de facto occupiers whose money is drying up now that the international mission is winding down.

The hospital attacker, who has not been identified, served in the Afghan Public Protection Force and was assigned as a guard at the facility, District Police Chief Hafiz Khan said. The APPF is an armed security force under the Interior Ministry that was created to protect foreign organizations.

According to its website, the Cure International Hospital was founded in 2005 by invitation of the Afghan Health Ministry. 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.

Umanos, the slain doctor, “was always working to help inner-city kids and trying to help out any needy, poor kids anywhere,” Jeff Schuitema, his brother-in-law, said.

“Our families and friends have suffered a great loss, and our hearts are aching,” Jan Schuitema, Umanos’ wife, said at the family home in Chicago. “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.”

Mark Knecht, Cure International’s chief financial officer, told reporters outside the group’s headquarters in Lemoyne, Pennsylvania, that it “remains committed to serve the people of Afghanistan.”