These are the victims of the shooting at Chicago's Mercy Hospital

Three victims, including a beloved emergency room doctor, a first-year pharmacy resident and a young police officer, are dead after a gunman opened fire at a hospital on Chicago's South Side Monday afternoon.

Authorities say Dr. Tamara O'Neal, 38, was shot dead by her former fiancee, 32-year-old Juan Lopez, outside of Mercy Hospital and Medical Center on Nov. 19 following an argument.

The Associated Press reports that the shooting is being investigated as a domestic dispute between Lopez and Dr. O'Neal, who is believed to be the intended target of the attack.

"It was the end of her shift," said Patrick Connor, director of emergency medicine and chair of the emergency department at Mercy. "She had no children. Never missed work. The best person ever, really."

After shooting his former fiancee dead in the parking lot of the hospital, Lopez entered the building and continued rapidly firing his weapon, killing first-year pharmacy resident Dayna Less and Chicago Police Officer Samuel Jimenez. Lopez was also found dead from a gunshot wound, although it remains unclear if he was killed by police fire or if he turned his weapon on himself. 

Less, a 25-year-old recent graduate of Purdue University, was struck by a bullet as she walked out of an elevator inside of the hospital. She was engaged to her high school sweetheart and was set to be married on June 1 in Indiana.

"That poor woman that got off an elevator had nothing to do with nothing, and he shot her," said Chicago police Superintendent Eddie Johnson. "Why?"

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Victims of shooting at Mercy Hospital in Chicago
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Victims of shooting at Mercy Hospital in Chicago

Dr. Tamara O'Neal, 38, was shot dead by her former fiancee, 32-year-old Juan Lopez, outside of Mercy Hospital on Nov. 19, 2018.

(Photo: Franciscan Health)

Dayna Less, 25, a first-year pharmacy resident, was struck by a bullet as she walked out of an elevator inside of the hospital.

(Photo: Facebook)

Samuel Jimenez, a Chicago police officer on the force less than two years, was gunned down as he went to the aid of other officers during the shooting.

(Photo: Chicago Police Department)

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Officer Samuel Jimenez, 28, was gunned down as he rushed to the aid of other first responders. Jiminez, a rookie officer who had recently completed his probationary period, was married and had three small children. 

According to Superintendent Johnson, Jiminez and his partner were not assigned to respond to Mercy Hospital, but still rushed to the scene when they heard there was an officer in distress because "that's what we do."

"When they pulled up, they heard the gunshots, and they did what heroic officers always do — they ran toward that gunfire," Johnson said.

"Those officers that responded today saved a lot of lives," he added. "They were heroes because we just don't know how much damage (the shooter) was prepared to do."

Chicago Mayor Rahm Emanuel condemned the violent attack during a Monday press conference. 

"The city of Chicago lost a doctor, a pharmaceutical assistant and a police officer all going about their day, all doing what they love," he said. "This tears at the soul of our city. It is the face and the consequence of evil."

Michael Davenport, Mercy's chief medical officer, said the hospital held its first active shooter drill just weeks before the incident, which he believes may have prevented even more senseless bloodshed.

"Never in our wildest imagination would we ever think that we would have to experience the day we have," he said. "It is our inclination, I can speak for myself, you don’t feel well when you’re hiding. You want to open up a door and you want to see what’s going on and you want to help. But everyone did what they were trained to do."

RELATED: Photos from the scene: 

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Deadly shooting at Mercy Hospital in Chicago
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Deadly shooting at Mercy Hospital in Chicago
Chicago Police officers walk outside Mercy Hospital on the city's South Side where authorities say a shooting at the hospital has wounded multiple people, including a suspect and a police officer, Monday, Nov. 19, 2018, in Chicago. (AP Photo/Amanda Seitz)
Employees from the hospital ride a Chicago Transit Authority bus after being evacuated from Mercy Hospital where shots were fired Monday, Nov. 19, 2018, in Chicago. A police spokesman said the gunman was dead, but it was not immediately clear if he took his own life or was killed by police at the hospital on the city's South Side. (AP Photo/David Banks)
Chicago Police work the scene after a gunman opened fire at Mercy Hospital, Monday, Nov. 19, 2018, in Chicago. A police spokesman said the gunman was dead, but it was not immediately clear if he took his own life or was killed by police at the hospital on the city's South Side. (AP Photo/David Banks)
A Chicago police officer works the scene after a gunman opened fire at Mercy Hospital, Monday, Nov. 19, 2018, in Chicago. A police spokesman said the gunman was dead, but it was not immediately clear if he took his own life or was killed by police at the hospital on the city's South Side. (AP Photo/David Banks)
Chicago Police work the scene where a gunman opened fire at Mercy Hospital, Monday, Nov. 19, 2018, in Chicago. A police spokesman said the gunman was dead, but it was not immediately clear if he took his own life or was killed by police at the hospital on the city's South Side. (AP Photo/David Banks)
Chicago Police work the scene after a gunman opened fire at Mercy Hospital, Monday, Nov. 19, 2018, in Chicago. A police spokesman said the gunman was dead, but it was not immediately clear if he took his own life or was killed by police at the hospital on the city's South Side. (AP Photo/David Banks)
Chicago Police work the scene after a gunman opened fire at Mercy Hospital, Monday, Nov. 19, 2018, in Chicago. A police spokesman said the gunman was dead, but it was not immediately clear if he took his own life or was killed by police at the hospital on the city's South Side. (AP Photo/David Banks)
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