Updated at 11:55 a.m. Tuesday CST

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 resident 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.

It was unclear if he took his own life or was killed by police, Chicago Police Superintendent Eddie Johnson said late Monday outside another hospital, just minutes after leaving the slain officer’s family.

“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.

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.”

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.

Today, we mourn Chicago Police Officer Samuel Jimenez. His heroic actions saved countless lives. He ran toward danger. He ran toward those shots. He ran into fire. Selflessly.

Samuel Jimenez: altruism personified. #OfficerDown #NeverForget pic.twitter.com/7IDW4sJr6v

— Chicago Police (@Chicago_Police) November 20, 2018

Mercy Hospital identified one of the slain staff members as 38-year-old Tamara O’Neal, an emergency room physician who never worked on Sunday because of her religious faith.

I also have a mutual friend of Dr. O’Neal who is devastated by her loss. The world is smaller than we know and acts of violence affect all of us.#Chicago#FifthDistrict #Enough https://t.co/hbyn9HwEg1

— Juliana Stratton (@RepStratton5)November 20, 2018

The second hospital staffer was Dayna Less, a 25-year-old a first year pharmacy resident who had recently graduated from Purdue University. Less was engaged to be married in 2019. WBEZ’s Michael Puente spoke with Less’ father Tuesday.

Attorney Brian Less of St. John, Indiana, father of Dayna Less, speaks about his daughter, one of the four victims in Monday’s shooting at Mercy Hospital in Chicago. Dayna was a Lake Central HS & Purdue graduate. pic.twitter.com/TYVQsygtrx

— Michael Puente (@MikePuenteNews) November 20, 2018

I was a close friend of Dayna Less. One of her and her family’s favorite things together was celebrating their love for the Cubbies. ♥️ Please pray for her Mother, Father, and Fiancé during this horrific time. pic.twitter.com/4bvreD32gS

— Nina Starcevich (@nina_starcevich) November 20, 2018

According to the Chicago Tribune, Lopez was accused of “aggressive and improper conduct towards women” at the Chicago Fire Academy five years ago.

.@theCHAtweets releases statement about Juan Lopez, the offender in Mercy Hospital shooting who worked as a CHA associate program specialist since Feb. ‘unthinkable act of violence. We are filled with overwhelming sadness…’

— Natalie Y Moore (@natalieymoore) November 20, 2018

What happened?

Chicago Police Department spokesperson Anthony Guglielmi described the shooting as a “domestic-related active shooter incident,” but provided no details about the relationship between the two.

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. After O’Neal fell to the ground, Lopez “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 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. She looked down and saw blood on the floor but no bodies.

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

Maria Correa hid under a desk, clutching her 4-month-old son, Angel, while the violence unfolded. 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,” Correa said

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, when the Sisters of Mercy religious group converted a rooming house. During the Civil War, the hospital treated both Union soldiers and Confederate prisoners of war, according to its website.

Associated Press Writer Michael Tarm and digital producer Gabrielle A. Wright contributed to this report.

Follow WBEZ producer Arionne Nettles on Twitter for the latest updates.