The shootings began with an argument in the Mercy Hospital auto park between the gunman, 32-year-old Juan Lopez, and Dr O'Neal at about 15:00 local time on Monday.
Once inside the hospital, Lopez shot pharmacy resident Dayna Less as she exited an elevator.
That came after the worst anti-Semitic attack in modern USA history, when a gunman opened fire on worshipers at a synagogue in the United States city of Pittsburgh on October 27.
During the initial rampage, he also shot Chicago Police Officer Samuel Jimenez, 28, a first-year rookie and father of three, fatally wounding him.
The name of the shooter was not revealed. "And she said, 'Oh, just tell him I'm in with a patient'".
Dayna Less Facebook
"The offender lifted up his shirt and displayed a handgun", Superintendent Johnson said.
The friend ran into the hospital to call for help, and the gunfire began seconds later.
First responders arrive to Mercy Hospital where multiple people were shot, including a Chicago police officer, Monday, Nov. 19, 2018, in Chicago.
The hospital paid tribute to police and security staff, saying it was "deeply saddened by the tragedy".
At 11:57 a.m., the 15-year-old was riding in a vehicle in the 100 block of North Pulaski when a blue sedan pulled up and someone inside started shooting, according to police.
Another witness, who said she worked at a building next to the hospital, heard five shots.
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"[Dr Less] had nothing to do with nothing", he said. "It was just like we're talking now", he said.
Samuel Jimenez was killed Monday by a gunman at Mercy Hospital.
During the exchange of gunfire, a squad vehicle was struck and a bullet became lodged in the gun of another officer.
Inside the hospital there were scenes of chaos as he fired at will and managed to reload twice. Officers weren't assigned to the particular call, "but they went because that's what we do", Johnson told reporters. A few inches either way, and the officer would have been severely wounded.
The gunman, whose name has not been released, also died, officials said.
Mayor Rahm Emanuel called it a devastating moment for a city that for years has endured daunting levels of gun violence. "It is the face and effect of evil", Emanuel said.
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Superintendent Jackson said that O'Neal, an emergency room doctor, had broken off an engagement with the suspect several months ago.
Jimenez was rushed to University of Chicago Medical Center, where he was later pronounced dead, authorities said. "Tonight, I broke down in front of my coworkers when we lost her, and tonight I held hands with her mother in prayer".
Many in the medical community took to social media to share their sentiments over the tragic loss of the 38-year-old ER physician. He was married with three children.
"I don't know what happened", the unnamed employee told the Tribune.
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