FOUR people have died including a hero police officer, an A&E doctor and a pharmaceutical assistant after a gunman stormed a hospital on Monday.

The suspect first gunned down a woman in the car park before running inside Mercy Hospital in Chicago, Illinois, and opening fire at around 3.30pm local time.

Officers raced inside where they engaged in an exchange of gunfire with the suspect, who fatally shot a female employee as she exited a lift, police said.

Police officer Samuel Jimenez, 28, who had recently passed his probation, was also shot and taken to another hospital where he later died.

The shooter was also killed, however, officers say it’s unclear at this stage whether the man died from a self-inflicted injury or from a bullet by cops.

Local reports have named the woman slain in the car park as the man’s alleged ex-fiance, Dr Tamara O’Neal.

Superintendent Eddie Johnson said the tragedy started in the car park where the suspect and the doctor, who were in a relationship, were having an argument.

He said a friend of one of the individuals attempted to intervene in the dispute and the offender lifted up his shirt to display a handgun.

Cops say the acquaintance ran inside to make a 911 call before a second call was made confirming shots had been fired in the car park.

Officers rushed to the scene and the suspect fired multiple bullets at a police car, before running inside the hospital.

Johnson said the officers gave chase and engaged the offender for several minutes with gunshots fired between both police and the suspect.

A female member of staff, who Mayor Rahm Emanuel said was a pharmaceutical assistant, was fatally shot by the suspect and officer Jimenez was also killed.

Johnson added: “That poor woman got off an elevator. She had nothing to do with nothing and he shot her. Why? We just don’t know how much damage he was prepared to do.”

The tragic young officer, who was married with three kids, had been on another job nearby when he and his partner responded to the assistance call.

Johnson said: “They did what heroic officers do – they ran towards the gunfire.

“They weren’t assigned to that call but they went because that’s what they do.”

He also confirmed a second officer was shot in his gun holster.

“The city of Chicago lost a doctor, a pharmaceutical assistant and a police officer, all going about their day, all doing what they love,” Chicago Mayor Rahm Emanuel told reporters.

“This tears at the soul of our city. It is the face and consequence of evil,” Emanuel said.

Witnesses described the chaos as patients, workers and visitors were evacuated from the facility.

Eyewitness James Gray told ABC7 Chicago: “It was chaos, mass chaos.”

He described seeing a man and a woman who looked like they were talking to each other while walking towards the car park.

Mr Gray said: “The gentleman turned around and shot three times in the chest. Once she fell to the ground, he turned around and shot her three more times.”

He said he didn’t know if they knew each other, and added: “They were walking and talking and he just turned around and started shooting. It wasn’t a heated exchange.”

Witness Steve White, who was inside the emergency room, told CBS: “We heard some shots so we all ran to the emergency window, we see this guy out there, military style, shooting at the police.

“He goes pow, pow, pow, and then he turns around and whoever was laying on the ground in hospital scrubs and shoots them twice… pow, pow.

“The he runs into the hospital, we were told to stay down and then we heard more shots… pow, pow, pow.”

“We heard more shots and we were being told to stay calm.”

He added: “It was just one gunman and he didn’t care about nobody.

“People were hollering, screaming, scared. I lost my wallet in there but they can have it.”

The hospital is not a trauma centre, so the severely injured were transported to another nearby facility.