KABUL, Afghanistan (CNN) -- A Chicago pediatrician who "felt called" to move to Afghanistan to treat children and train physicians was among three Americans killed at a Kabul hospital Thursday by an Afghan guard.
In an attack not unlike the so-called "green-on-blue" incidents, the police guard opened fire on the pediatrician and four others with him at the CURE Hospital's gates, Kabul police said.
Two others were killed, a third was injured, and a fourth person was unharmed.
Dr. Jerry Umanos practiced medicine in inner-city Chicago before moving to Afghanistan in 2005, according the U.S. hospital he was affiliated with, Lawndale Christian Health Center.
Umanos was affiliated with LCHC for more than 25 years, said Dr. Bruce Rowell, the hospital's chief clinical officer.
"He was a loving, caring physician" who served his patients "with the utmost of respect," he said.
Umanos worked at the Kabul hospital as well as at a community health center, the only two training programs for Afghan doctors in the country, according to his bio at LCHC's website.
"He loved the country, he loved the people, he loved to teach," a former colleague told CNN. "As much as we love and miss him, all of us have a certain level of respect and contentment knowing Jerry died doing what he loved most."
Umanos' wife and grown children, who live in the United States, have been notified of his death, the source said.
The senior pediatrician had worked at the hospital in Kabul for seven years, longer than anyone else.
Two of those killed Thursday were father and son, Afghan Minister of Public Health Suraya Dalil said.
The injured victim has undergone medical treatment and is in stable condition, she added.
The police guard shot himself but survived, police spokesman Hashmatullah Stanekzai said. The motive for the attack was not immediately clear.
The U.S. Embassy in Kabul confirmed the shooting.
CURE is a non-profit organization that operates hospitals and programs in 29 countries, according to its website.
It said patients "experience the life-changing message of God's love for them" while receiving treatment regardless of gender, religion or ability to pay.
The CURE hospital in Kabul has about 100 beds and about 37,000 patients annually.
In Chicago and Afghanistan alike, Umaros knew that patients' circumstances meant that many could not return for follow up visits, the website said.
"In both places, he knows that he must provide the best care possible at each visit, because there is a chance that he may not see the patient again," the bio states.
More attacks on foreigners
Afghanistan has seen a spate of deadly attacks against foreigners in recent weeks.
On April 4, two Associated Press journalists were shot in Afghanistan's eastern Khost province. Award-winning German photographer Anja Niedringhaus was killed, and Canadian reporter Kathy Gannon was injured.
And last month, five militants set off a deadly car bomb and then stormed a guest house used by foreigners in Kabul, the Afghan interior ministry said.
The militants held several foreigners hostage, including three Americans, a Malaysian and a person from an unspecified African country.
One girl was killed in the bombing. By the end of the hostage ordeal, One of the militants was shot and killed, and the other four blew themselves up.
The Taliban claimed responsibility for the March attack. Taliban spokesman Zabiullah Mujahid said militants targeted a location that foreigners used as a church and for converting Afghans to Christianity.
CNN's Mariano Castillo and Holly Yan reported and wrote from Atlanta; Qadir Sediqi reported from Kabul. CNN's AnneClaire Stapleton and Brian Walker contributed to this report.