The man who shot and killed three people Monday at Mercy Hospital and Medical Center on South Side, Chicago, including a police officer, a doctor and a pharmacist, was identified as Juan Lopez.

Lopez, 32, was the former fiancée of Tamara E. O’Neal, an emergency room doctor at the hospital, who was seen arguing with the shooter in the parking lot of the hospital, prior to the shooting. The two were supposed to get married Oct. 27. It was unclear as to when and why their relationship ended.

After graduating from University of Illinois College of Medicine in Chicago, O'Neal started her career with the College of Medicine’s emergency medicine residency program. She also worked at the Franciscan Health Michigan City in Indiana. Lopez’s ex-fiancée was quite excited about the possibilities of emergency medicine.

“You never know what is going to happen. You get to impact so many people from so many walks of life. You get to save lives,” O’Neal said, Heavy reported. “I’m so overwhelmed by so many emotions. I’m so grateful to God, my family and my EM mentors. I know my future is going to be great! By training in the UIC emergency medicine program, I know I’m going to be the best physician I possibly can be.”

On the day of the shooting, O’Neal had reported for her shift at the hospital before being confronted by Lopez outside the hospital. Following a heated argument between the two, which was witnessed by at least one passerby, Lopez pulled out a gun and shot O’Neal multiple times at around 3:30 p.m. local time (4:30 p.m. EST).

Witnesses saw Lopez continue to fire shots at his ex-fiancée even after she fell on the ground. Meanwhile, one of O’Neal’s friends, who witnessed the shooting, went inside the hospital, trying to get help.

The shooter then proceeded to fire a few bullets at the police squad car which arrived at the scene, before running into the hospital. He continued his shooting spree, killing a pharmaceutical assistant whose name has not been released and fatally wounding Chicago Police Officer Samuel Jiminez, who was pronounced dead after being rushed to a hospital.

Several people present at the scene at the time of the shooting described the horrific situation where patients, staff and visitors were told to take cover.

In a press conference following Jiminez’s death, Chicago Police Superintendent Eddie Johnson said at a press conference Lopez was dead, although it was not immediately clear whether he was shot by police officers or he died from self-inflicted wounds. The shooter was believed to be acting alone as a SWAT team searched the area for additional shooters and did not find anyone else.