CHICAGO—A gunman killed a doctor at Mercy Hospital as part of a domestic dispute on Monday, authorities said, before bringing his rampage inside where he killed two other people, including a police officer.

Dr. Tamara O’Neal, Officer Samuel Jimenez, and a pharmaceutical assistant were killed by a man wielding a pistol, Chicago Police Superintendent Eddie Johnson said at a press conference Monday night.

The gunman got into a “verbal dispute” with a woman in the hospital’s parking lot, Johnson said, before opening fire. An eyewitness told local media the gunman fired three shots at the woman before pausing and firing three more shots. The Chicago Tribune , citing sources, reported O’Neal was confronted by the gunman over a “broken engagement.”

A coworker at the scene described O'Neal to The Daily Beast as a “warm and well liked” critical-care doctor who worked in the hospital’s intensive care unit.

After shooting O'Neal outside, t he gunman moved inside the hospital where he opened fire, striking the pharmaceutical assistant. Officer Jimenez was shot as he responded to the scene and was found inside the hospital. Jimenez, 28, a father of three, had been on the force less than two years.

Eyewitnesses reported hearing gunshots inside the hospital. Two women, Monique Hubbard and Jennifer Eldridge, told reporters the gunman tried to open a locked door to the pharmacy where they were sheltered, but he did not enter. Blood was seen strewn on the floor, other eyewitnesses said.

Johnson said the perpetrator was fatally shot, but added it was unclear if he killed himself or was killed by police.

An occupational therapist working inside said she first heard about the shooting when the hospital issued an alert over its public-address system.

“Then people began running around saying to shut doors and barricade them and hide under desks,” the therapist told The Daily Beast. “It was terrifying. I had to look on my badge to see what ‘code silver’ was and then I realized it was serious.”

Mayor Rahm Emanuel, choking back tears at the press conference, said the victims were slain while doing what they loved.

“This tears at the soul of our city. It is the face and the consequence of evil,” Emanuel said.