According to the U.S. Embassy in Afghanistan, Three American doctors who worked for a Christian relief organization were shot to death by an Afghan security guard who opened fire at a Kabul hospital.

"With great sadness we confirm that three Americans were killed in the attack at CURE Hospital," said a statement also on the Embassy's Twitter page. "No other information will be released at this time."

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

One of the doctors has been identified as Dr. Jerry Umanos, who practiced pediatric medicine at Lawndale Christian Health Center in Chicago, officials from the center said.

According to the Chicago Tribune, Umanos had spent more than a decade training doctors and tending to the poor in the war-torn country. But lately he was growing more worried about his safety.

The pediatrician from Chicago had been shot at while jogging, and a dentist who lived in a neighboring compound had been killed in January.

"He was obviously concerned," his friend and colleague, Dr. Art. Jones told the tribune. "At the same time, you can't count the number of children that Jerry's impacted, the lives he's saved on his own, and with the doctors he trained. That's who he was. He was driven by the kids."

Dalil said an American nurse also was 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, were killed," Dalil said. "The two visitors were father and son, and a woman who was also in the visiting group was wounded."

The alleged attacker was a member of the Afghan Public Protection Force assigned to guard the hospital, according to District Police Chief Hafiz Khan. At this point, they are unsure of his motive for the attacks.  This is the second "insider attack" by a member of the Afghan Police force this month.

According to the NY Times, An Afghan government official said the policeman,  Ainuddin, had only recently been assigned to the unit guarding the hospital. Witnesses and officials said he fired on the Americans when they entered a security vestibule at the entrance to the building, killing them and wounding a female doctor. Ainuddin, who like many Afghans goes by one name, then entered the interior courtyard, where he continued to fire, officials said.  The policeman was a two year veteran of the course.

"The shooter, who was not an employee of CURE, has been identified as a member of the security detail assigned to the hospital, shot himself after the attack," CURE Hospital said in a statement.  "He was initially treated at the CURE Hospital and has now been transferred out of our facility into the custody of the government of Afghanistan."

According to Cure International's website, the organization's mission is healing the sick and proclaiming the kingdom of God.

"Those words come directly from Jesus' own instructions to his disciples - first to the twelve and then to the seventy. The partnership of healing the sick and sharing the good news of "God with us" was linked in almost every facet of His life and work, and CURE strives to be a 21st century expression of Jesus' 1st century healing ministry," Cure's mission statement reads.  

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.

The attacker had emerged from surgery in the afternoon and was in recovery at Cure International before being questioned, Dalil added.

This is the latest in what is an alarming trend of attacks on foreign civilians in 2014.

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