The attack took place at Chicago's Mercy Hospital in the city at about 15:00 local time. The hospital was evacuated during the incident, reports said.

A police officer, a doctor, and a pharmaceutical assistant were killed in the attack.

The gunman, who was in a relationship with one of the victims, also died, Chicago Police Superintendent Eddie T. Johnson was quoted as saying by CNN.

It is still not clear whether the attacker was killed during an exchange of shots with the police or he took his own life.

Officials were quoted as saying by BBC that the gunman appeared to target a woman he was in a relationship with.

They, however, did not suggest a motive of the attack. 

The deceased police officer was identified as Samuel Jimenez.

"It's with profound sadness that we share the death of PO Samuel Jimenez from tonight's senseless active shooter incident. Please pray for his family, his fellow officers & the entire #ChicagoPolice Department. Supt Eddie Johnson & @ChicagosMayor will hv a press briefing shortly," tweeted Chicago Police spokesperson Anthony Guglielmi.

Speaking on the number of deaths in the incident, he said: "UPDATE - Three innocent victims and the cowardly gunman who deserves to be nameless were killed tonight."

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