A gunman repeatedly shot an emergency room doctor outside a Chicago hospital, including after she fell to the ground, before he ran into the building and fatally shot a responding police officer and a pharmacy worker as she walked out of an elevator, according to witnesses and investigators.

The attacker, Juan Lopez, also died during the shooting Monday at Mercy Hospital on the city's South Side, but it was unclear if he took his own life or was killed by police, Chicago Police Superintendent Eddie Johnson said.

"This officer, all of those officers are heroes, they saved a lot of lives because we just don't know how much damage he was prepared to do," Johnson said.

The hospital identified the slain staff members as 38-year-old Tamara O'Neal, an emergency room physician, 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.

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

Lopez, 32, and O'Neal had been arguing in the hospital parking lot just before the shooting. When one of O'Neal's friends 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.

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

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

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 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.

The death of Jimenez comes nine months after another member of the Chicago Police Department, Cmdr. Paul Bauer, was fatally shot while pursuing a suspect in the Loop business district.

Mercy has a rich history as the city's first chartered hospital. It began in 1852. During the Civil War, the hospital treated both Union soldiers and Confederate prisoners of war, according to its website.