Four people are dead, including an ER doctor and a police officer, after a gunman killed his ex-fiancee and two others at Mercy Hospital in Chicago on Monday.

The 32-year-old gunman, identified by sources as Juan Lopez, and his ex-fiancee Dr Tamara O'Neal are among the dead.

Chicago police officer Samuel Jimenez, 28, was also killed in the shooting. Jimenez, a married father-of-three, had only just completed his probationary period.

The second woman who was shot by Lopez was a pharmaceutical technician at Mercy Hospital. Her name has not been released.

O'Neal, an ER doctor at the hospital, was reporting for work when she was confronted by Lopez just before 3.30pm.

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Juan Lopez has been identified as the shooter who gunned down his ex-fiancee and killed two other people, including a police officer, at Mercy Hospital in Chicago on Monday

Dr Tamara O'Neal, an ER doctor, was killed by Lopez on her way to work at Mercy Hospital

Chicago police officer Samuel Jimenez, 28, (pictured) was also killed in the shooting. Jimenez had only just completed his probationary period. He was a father of three

Witnesses said Lopez shot O'Neal in the hospital parking lot before rushing inside and opening fire at others

Lopez, 32, died inside the hospital after he was reportedly shot by a SWAT team officer

The hospital was partially evacuated following reports of an active shooter and a stream of doctors in white coats and scrubs could be seen leaving Mercy

Witnesses said Lopez targeted O'Neal in the parking lot, shooting her at least six times before going inside the hospital

O'Neal and Lopez had reportedly planned to wed on October 27, but she called off the wedding in September.

There was a 'verbal altercation' between O'Neal and Lopez in the parking lot, Chicago Police Department Superintendent Eddie Johnson said during a press conference on Monday night.

A third person, who Johnson said was friends with 'one of the individuals', then attempted to intervene.

'At that time, the male individual lifted up his shirt and displayed a handgun,' Johnson added.

The acquaintance fled inside the hospital and 911 soon received a call of an assault taking place in the parking lot.

Just seconds later, a second call came in. Shots had been fired.

James Gray saw Lopez and O'Neal talking to one another in the parking lot when he suddenly pulled out a gun.

'The gentleman turned around and shot three times in the chest,' Gray told WLS. 'Once she fell to the ground, he turned around and shot her three more times'.

After Lopez shot O'Neal again, he shot at a police car that began driving towards him and then entered the hospital.

'Officers observed him with a handgun,' Johnson said. 'He fired multiple shots before they could exit the squad car.'

Officers did a 'methodical search' of the hospital (pictured is a SWAT team inside the hospital)

After Lopez shot O'Neal again, he shot at a police car that began driving towards him and then entered the hospital

Lopez shot a pharmaceutical assistant, who later died from her injuries, as well as Jimenez

The cops gave chase and put out an officers need assistance call, which Jimenez and his partner responded to.

'When they pulled up, they heard the gunshots,' Johnson said. 'They did what the heroic officers always do, they ran towards the gunfire.'

'They weren't assigned to that particular call, but they went.'

Lopez then shot the pharmaceutical assistant just as she was getting off an elevator.

'That poor woman got off an elevator,' Johnson said. 'She had nothing to do with nothing, and he shot her. Why?'

Lopez also shot at a second police officer, but the bullet struck his holster and embedded into the gun. He was not injured.

Gray said the scene inside the hospital was 'mass chaos'.

'Once he entered, he just started shooting at random,' Gray told NBC Chicago. 'It looked like he was turning and pointing at people at random.'

Lopez had also shot at a second police officer, but the bullet struck his holster and embedded into the gun (pictured). He was not injured

There was a massive police presence at Mercy Hospital after gunfire broke out on Monday

Witness Hector Aditia was watching television in the hospital's waiting room when he saw Lopez shoot O'Neal.

Aditia said Lopez reloaded twice, firing '32 bullets each' time, once he went inside the hospital.

'He was shooting like a maniac,' Aditia told the Chicago Tribune. 'And he obviously knows how to shoot because he was holding the gun with both hands.'

'I was trying to help out as much as I could.Get people out of the way of the windows. Because he could have easily just aimed at us too.'

Lopez died inside the hospital after he was shot in the head by a SWAT team officer, according to CBS Chicago.

Jimenez was taken to the University of Chicago Medical Center in critical condition and died hours later from his injuries.

A procession from the hospital to Cook County Morgue will be held for Jimenez on Monday night.

Employees and patients were put on Chicago Transit Authority buses (pictured) while police searched the hospital

Emergency vehicles could still be seen outside Mercy Hospital hours after the shooting

Police did a 'methodical search' of the hospital following the shooting and 15 ambulances arrived at the scene.

A hospital employee said a notice came over Mercy's public address system telling everyone in the hospital to lock their doors.

Nigary Thomspon, an employee of the hospital's family clinic, heard eight or nine shots. Employees and patients at the clinic locked the doors and hid in the back.

'I am scared as hell. I have never been so scared, I hear of shootings going on every day at people's workplaces but not where I work,' Thomspon told WLS.

'This was very close to me. That could have been us back there and if any bullets had pierced the wall we all would have been hurt.'

The hospital was later 'partially evacuated' and people were put on Chicago Transit Authority buses while officers searched Mercy.

'They told us to run, so we did,' another employee added.

A nearby high school and elementary school were also placed on temporary lockdowns. The shooting was officially declared over by the hospital at 4.40pm.

A hospital employee said a notice came over Mercy's public address system telling everyone in the hospital to lock their doors during the active shooter situation

Mercy Hospital wrote on Facebook that the Chicago Police Department had secured the hospital and that all patients were safe.

Mayor Rahm Emanuel paid tribute to the victims during a press conference on Monday night as he spoke of the shooting.

'The city of Chicago lost a doctor, a pharmaceutical assistant, and a police officer,' he said. '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.'

'I ask each of us to hold our children, our loved ones, closer, remember what is important in life, and that there are others who are part of our larger family who will have a tear and a hole that will never really heal, that will always have a scar.'

Johnson called Jimenez and his fellow responding officers heroes.

'Those officers that responded today saved a lot of lives because this guy was just shooting,' he said.

'We just don't know how much damage he was prepared to do.'