A doctor, a pharmacist and a police officer were killed after a gunman opened fire at a hospital in Chicago.

The shooter was seen arguing with the doctor, Tamara O'Neal, who he had been in a relationship with, before he shot her and turned his gun on others.

The shooter, who has not been named, also died but it is not clear if he took his own life or was killed by police at Mercy Hospital on the city's South Side.

Mercy Hospital said the staff who died were Tamara O'Neal, 38, an emergency room physician who never worked on Sunday because of her religious faith, and Dayna Less, 25, a first year pharmacy resident who had recently graduated from Purdue University.

The Herald:

The officer was father-of-three Samuel Jimenez, 28, who joined the department in February 2017 and had recently completed his probationary period, Chicago Police Superintendent Eddie Johnson confirmed.

The shooting began after an argument between Ms O'Neal and the gunman in the hospital car park.

When a friend of Ms O'Neal tried to intervene, "the offender lifted up his shirt and displayed a handgun," Mr Johnson said.

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The friend ran into the hospital to call for help, and the gunfire began seconds later, with the attacker killing Ms O'Neal.

After Ms O'Neal fell to the ground, the gunman "stood over her and shot her three more times," a witness named James Gray told reporters.

When officers arrived, the suspect fired at their squad car and then ran inside the hospital. The police gave chase.

Inside the medical centre, the gunman exchanged fire with officers and "shot a poor woman who just came off the elevator" before he was killed, Mr Johnson said, referring to pharmaceutical assistant Ms Less.

"We just don't know how much damage he was prepared to do," Johnson said, adding that Ms Less "had nothing to do with nothing".

The Herald:

Jennifer Eldridge was working in a hospital pharmacy when she heard three or four shots that seemed to come from outside.

Within seconds, she barricaded the door, as called for in the building's active shooter drills. Then there were six or seven more shots that sounded much closer, just outside the door.

"I could tell he was now inside the lobby. There was screaming," she recalled.

The door jiggled, which Ms Eldridge believed was the shooter trying to get in.

Some 15 minutes later, she estimated, a Swat team officer knocked at the door, came inside and led her away.

She looked down and saw blood on the floor but no bodies.

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"It may have been 15 minutes, but it seemed like an eternity," she said.

Maria Correa hid under a desk, clutching her four-month-old son, Angel, while the violence unfolded.

Ms Correa was in the waiting area of the hospital for her mother-in-law's doctor appointment when a hospital employee told them to lock themselves in offices.

She lost track of how many shots she heard while under the desk "trying to protect her son" for 10 to 15 minutes.
"They were the worst minutes of our lives," Ms Correa said.