A Chicago police officer died after he was "senselessly murdered" in a downtown building Tuesday afternoon, police said.

Commander Paul Bauer, of the 18th District, was chasing a suspect when he was "shot multiple times," Police Superintendent Eddie Johnson said at a news conference.

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Bauer, 53, had spotted a man matching the radio description of an armed suspect when he "engaged in an armed physical confrontation," around 2 p.m., Johnson said.

The superintendent said that while "any loss of life in this city is tragic, today is difficult."

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Anthony Guglielmi, Chicago police's chief communications officer, clarified early Wednesday that Bauer was on duty at the time he was killed. Earlier reports said Bauer was off-duty.

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After the shooting, Bauer was transported to Northwestern Memorial Hospital. Fox 32 Chicago reported the commander was shot in a stairwell of the Thompson Center in the city's downtown business center.

Sources told Fox News that police seemed to be responding to a robbery but it's unclear what the suspect was doing that prompted officers to want to speak with him.

Chicago Police guard the crime scene after an off-duty Chicago police officer was shot Tuesday afternoon. (Fox News)

A "person of interest" has been taken into custody, police said.

Chicago Mayor Rahm Emanuel said "Bauer stood for the highest ideals of our police department & our city—to serve and protect the people of Chicago."

Bauer was a 31-year veteran of the Chicago Police Department.

Fox News' Matt Finn and The Associated Press contributed to this report.