An emergency room doctor and two others were killed when Juan Lopez opened fire at a Chicago hospital following an argument.

Dr. Tamara O’Neal was arguing with her ex-fiancé at Chicago's Mercy Hospital on Monday when he pulled out a gun and shot her. According to witness Steve Mixon, an emergency room clerk, Lopez called asking for O’Neal earlier that day, but she blew him off.

“He called and asked to speak with his fiancee,” Mixon told The Chicago Tribune. “And she said, ‘Oh, just tell him I’m in with a patient.’”

When Mixon was leaving for work, he saw the fateful argument and attempted to intervene.

“I look up, and I see her being harassed by some gentleman. She was trying to avoid him and move around. And when she saw me, she waved for me to come that way,” he continued.

He didn’t make it to her before Lopez starting shooting. Mixon said Lopez shot O’Neal a few more times after she fell. James Gray, another witness, counted three additional shots.

After he shot O’Neal, Lopez entered the hospital and began to shoot indiscriminately.

"Once he entered, he just started shooting at random," Gray told NBC Chicago. "It looked like he was turning and pointing at people at random."

When police officers arrived at the scene, the gunman turned his attention to them. A shootout ensued, and Officer Samuel Jimenez was fatally wounded. Dayna Less, a pharmacy assistant, was killed as she exited an elevator during the battle.

Lopez was later found dead of a gunshot wound to the head. It is unclear who shot him.

O’Neal was an emergency room physician who was dedicated to her work, her church and making the world a better place according to those who knew her.

"She was 38 years old, completely dedicated to her church," Patrick Connor, chairman of the emergency department at Mercy Hospital, told ABC7. "She never worked on Sundays. She led her choir in her church, and she fundraised every year for disadvantaged children for first, second and third-grade children all their school supplies."

"She’s one of the sweetest nieces that I have," O'Neal's aunt, Delores Davis, said. "She was also dedicated to her church."

Mixon remembered the young doctor as “a sweetheart, just a sweetheart. What a fireball.”

John Purakal, an emergency doctor at the University of Chicago Medical Center, knew O’Neal and was one of the physicians who tried to save her life. Her death had a profound effect on him.

This shooting comes two weeks after the NRA told doctors to “stay in their lane” regarding the conversation around gun violence in this country. Doctors across the country didn’t forget about those harsh words.

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ChicagoDr. Tamara O'NealJuan LopezMercy Hospital Shooting