Photo: The Canadian Press

Police salute as an ambulance arrives at the medical examiner's office carrying the body of Chicago Police Department Officer Samuel Jimenez, who was killed during a shooting at Mercy Hospital earlier in the day, Monday, Nov. 19, 2018, in Chicago.

An argument outside a Chicago hospital turned deadly when a man pulled out a gun and killed an emergency room doctor whom he knew, then ran into the hospital and fatally shot a pharmacy resident and a police officer, authorities said.

The attacker also died Monday but it was not clear if he took his own life or was killed by police at Mercy Hospital on the city's South Side, Chicago Police Superintendent Eddie Johnson said.

Chicago "lost a doctor, pharmaceutical assistant and a police officer, all going about their day, all doing what they loved," Mayor Rahm Emanuel said, fighting back tears. "This just tears at the soul of our city. It is the face and a consequence of evil."

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 slain officer was identified as Samuel Jimenez, 28, who joined the department in February 2017 and had recently completed his probationary period, Johnson said. Police said he was married and the father of three children.

The identity of the gunman was not immediately released.

Police spokesman Anthony Guglielmi described the shooting as a "domestic-related active shooter incident," but provided no details about the relationship between the two.

The gunman and O'Neal had been arguing in the hospital parking lot. When a friend of O'Neal tried to intervene, "the offender lifted up his shirt and displayed a handgun," Johnson said.

The friend ran into the hospital to call for help, and the gunfire began seconds later. After 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 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, Johnson said, referring to pharmaceutical assistant Less.