Chicago shooting: Police officer among four killed after gunman goes on rampage in Mercy Hospital

'I heard shots fired, I turned around to my right and I see a man shooting someone on the ground – shot him multiple times'

At least four people have died – one of them a police officer – after a gunman opened fire inside a Chicago hospital, officials said.

Officers rushed to the Mercy Hospital amid reports of multiple casualties after a suspect entered the facility on the city’s South Side and started firing. The 32-year-old suspect was killed during the incident, according to CNN, although it was not immediately clear if he had taken his own life or was shot by police. Some unconfirmed reports said his primary target was his ex-fiancee.

A police officer, Samuel Jimenez, died in the attack, a police spokesman said on Twitter. Two others who were struck by gunfire were taken to hospital in a critical condition but later died, an investigator at the Cook County Medical Examiner's Office confirmed.

Anthony Guglielmi, chief communications officer for the Chicago Police Department, tweeted for people to “avoid area. Heavy police response incoming”.

A police official told NBC News that the hospital had been partially evacuated, while officers searched the premises. Some people were hiding in locked rooms. A nearby school was also placed on lockdown for a period, though students were later allowed to go home.

A Twitter user posted what looked like footage of police officers searching the inside of Mercy Hospital and Medical Centre, which was first established in 1852, as they looked to clear the area.

CBS said sources reported that the gunman had targeted a woman, who witnesses said was dressed in hospital scrubs, while she was in the parking lot of the hospital.

“I heard shots fired, I turned around to my right and I see a man shooting someone on the ground – shot him multiple times,” the witness said.

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“Then almost immediately an officer was already coming in … He exchanged fire with them, and then reloaded, shot the person on the ground again, and then made himself inside the hospital and more shots fired.”

Mr Guglielmi said officers had responded to reports of multiple victims. At least 15 medic units were sent to Mercy Hospital, according to Chicago Fire Department public information officer Larry Merritt.

Erix Horton, who works for the hospital’s environmental services department, told the Chicago Tribune of the moment when he was told of the shooting

“I was checking out, getting ready to leave,” said Mr Horton. “One of the nurses ran back here and it was like she was about to collapse and said (a staff member had) been shot. And she’s like, ‘Call the police. We have an active shooter.’ And that¹s when everybody took cover. They got on the PA, letting everybody know.”

The newspaper said the first reports of shots came at 3.20pm. By 4.40pm, the hospital was able to tweet that the premises had secured and that patients were safe.

“A shooting took place at Mercy Hospital & Medical Centre this afternoon,” it said. “The shooting at Mercy Hospital is over. Chicago Police Department have secured the hospital and patients are safe.”

Mr Horton said he took cover with others in the break room that had a door that could be locked until police entered and escorted people out. A Fire Department crew that had just brought in a patient took cover in the room also, Mr Horton said.

While in the break room, they could hear someone firing gunshots in the hallway, he said.

“We had to duck,” he said. “A lot was going through my mind,” he said. ”Make it home to see my wife and my kids.”

A spokesman for Mayor Rahm Emanuel says the mayor and Police Superintendent Eddie Johnson are monitoring the situation.

Police have not commented on a possible motive for the shooting.