Four dead after Chicago hospital gunman opened fire, killing ex-fiancee

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 this morning.

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 Daily Mail reported.

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 local time.

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

Dr Tamara O'Neal, an ER doctor, was killed on her way to work at Mercy Hospital on Chicago's South Side by her ex-fiance Juan Lopez, who also shot two others at the hospital.

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 Tuesday morning (NZT).

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

Chicago police officer Samuel Jimenez, 28, (pictured) was also killed in the shooting. Photo / Supplied

"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."