CHICAGO — An officer and three others, including the gunman, are dead after a shooting at Chicago's Mercy Hospital, police said Monday. Chicago Mayor Rahm Emanuel said a doctor and a pharmaceutical assistant were the other victims.

Anthony Guglielmi, a Chicago police spokesman, identified the slain officer as Officer Samuel Jimenez.

Guglielmi said the gunman was killed, but it's unclear if he took his own life or was killed by police. In a later tweet, Guglielmi called it a "domestic-related active shooter."

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Police Superintendent Eddie Johnson said the incident began in the parking lot with an argument between a man and a female hospital employee he was in a relationship with. A friend of the woman tried to intervene, and police said that is when the gunman lifted up his shirt and showed a gun. The friend fled into the hospital.

Johnson said they received a 911 call about the altercation in the parking lot, and within seconds, they received a second call about shots fired in the parking lot. Johnson said the gunman shot and killed the woman.

Chicago Police Officer Samuel Jimenez seen in a photo provided by the police department. Chicago Police Dept. via CBS Chicago

The suspect then fired several shots at responding officers before they could exit the squad car, and several of those shots caused damage to the car. Johnson said the gunman then ran into the hospital while being chased by officers.

Once inside the hospital, the gunman and the officers exchanged gunfire for several minutes, Johnson said. A female employee was shot and killed in the exchange of gunfire, Johnson said. Jimenez was shot and killed by the gunman.

A massive law enforcement presence was seen on the campus and medical staff were seen being evacuated after the first reports of the shooting. Guglielmi had urged citizens to avoid the area. The hospital tweeted the shooting is over and said Chicago Police Department has secured the facility and the patients are safe.

Witness Hector Avitia told CBS Chicago he was inside the hospital waiting for test results for his wife when he saw a gunman outside in the parking lot, dressed in black, fire on someone on the ground multiple times.

Avitia said an officer arrived on the scene in an SUV and the gunman fired at the officer. He said he saw the gunman reload, fire again at the person on the ground and then go inside the hospital where more shots were fired.

He said the gunman was carrying a handgun with multiple clips.

Witness Steve White told CBS Chicago he was waiting in the emergency room when he saw the gunman outside firing on a person on the ground, who he described as a woman wearing hospital scrubs.

"He was like, he didn't care about nobody, or nothing," White said.

He said the gunman ran inside, and he could hear more shots fired. White said a police sergeant ran into the emergency room with his gun drawn and told everyone to evacuate amid a chaotic scene.

"Why would you go in a hospital and kill some innocent people, man?" White said. "I've never seen anything like this in my life."