(Image source from: NDTV)

A gunman opened fire Monday at a hospital in Chicago, killing three people, including a police officer and two hospital worker, authorities said.

Speaking at a press conference Monday evening, Superintendent of the Chicago Police Department Eddie Johnson said four people died in the attack on Mercy Hospital, including the suspected gunman.

"It's been a very difficult day, for not just the Chicago Police Department, but for the city of Chicago," he said.

A doctor and a pharmaceutical assistance were also killed, along with Police Officer Samuel Jimenez.

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

The procession for Ofc. Jimenez has begun. CPD and our federal and state partners pay our final respects to a hero. pic.twitter.com/f28V2x8l9G

— Chicago Police (@Chicago_Police) November 20, 2018

While Jimenez was not initially assigned to the incident, he arrived on the scene shortly after getting a call for assistance. He later died of his injuries, according to Johnson.

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"It is with profound sadness that we share the death of PO Samuel Jimenez from tonight senseless active shooter incident," police spokesperson Anthony Guglielmi said in a tweet.

It's with profound sadness that we share the death of PO Samuel Jimenez from tonight's senseless active shooter incident. Please pray for his family, his fellow officers & the entire #ChicagoPolice Department. Supt Eddie Johnson & @ChicagosMayor will hv a press briefing shortly pic.twitter.com/2onFeyaSDK

— Anthony Guglielmi (@AJGuglielmi) November 20, 2018

Jimenez had been on the force since last February 2017, according to Johnson. He is survived by three children.

Police have referred to the shooting as a "domestic-related" incident. It apparently began with a "verbal altercation" in a parking lot "between individuals who knew each other," Johnson said. "The male individual shot and killed the female that he was in a domestic relationship with."

Witness James Gray recalled seeing the shooter walking with a woman before turning and firing multiple shots at her, the Associated Press reports. According to Gray, the shooter entered the hospital and opening fire "at random" around 3:20 p.m. Monday afternoon.

"It was chaos, mass chaos," he told KBC-TV.

Mercy Hospital, located on Chicago's South Side, said around 4:40 p.m. that the incident was under control and police "have secured the hospital and patients are safe," according to a statement.

-Sowmya Sangam