A Chicago police officer was fatally shot at Mercy Hospital on Monday, police confirmed.
Officer Samuel Jimenez was identified as the officer who was killed "from tonight's senseless active shooter violence," Anthony Guglielmi, Chief Communications Officer for the Chicago Police Department, tweeted. "Please pray for his family, his fellow officers & the entire #ChicagoPolice Department."
Chicago Police Superintendent Eddie Johnson said at a news conference that three people were killed in the shooting: Jimenez and two female staff employees of the hospital, one of whom police said was in a relationship with the shooter.
The gunman was also killed, although it was not immediately clear whether the shooter was killed or died from a self-inflicted gunshot wound.
Johnson said the incident began just after 3:30 p.m. when the gunman and a woman were arguing in the parking lot outside of the hospital. A third person, who was with the two, tried to intervene in the argument, which is when the gunman lifted up his shirt and displayed a handgun, according to police.
The gunman then ran inside of the hospital, where he fired shots. When Jimenez, who was "doing something else," heard the gunshots on the scene, he ran toward the noise. Officers weren't assigned to the particular call, "but they went because that's what we do," Johnson told reporters.
Chicago Mayor Rahm Emanuel said Jimenez was "selfless in his actions," and added the city "lost a doctor, pharmaceutical assistant and a police officer" in the shooting.
"All [were] going about their day. All [were] doing what they love," Emanuel said. "This tears at the soul of our city."
Jimenez was an officer from the department's second district, officials said. He joined the force in February 2017.
Earlier Monday, Guglielmi said "multiple victims" were reported at the hospital. He tweeted that, "Officers are doing a methodical search of Mercy hospital," and noted, "At least one possible offender is shot."
The hospital confirmed just after 5:30 p.m. that a shooting took place, and was over.
"Chicago Police Department have secured the hospital and patients are safe," the hospital tweeted.
The FBI is assisting Chicago police in their investigation.
Fox News' Matt Finn and Jake Gibson contributed to this report.