Bauer, a 31-year veteran, was in the area, overheard police radio transmission and confronted the individual, Johnson said.
Chicago Police Superintendent Eddie Johnson (front) prepares to speak to the press outside Northwestern Memorial Hospital following the shooting death of Cmdr.
Guglelmi says police have taken a person into custody for questioning but he did not know if the person is a suspect. Gugliemi said on Twitter the officer was shot by "an assailant". Paul Bauer was shot several times.
The dispatcher said, "OK, is that an off-duty PO (police officer)?"
Moments later, officers discovered Bauer in a stairwell. It wasn't immediately clear if the shooting occurred inside or outside the building.
Chicago police officer fatally shot at downtown building
'And everybody is scurrying all over and I see the police head to the stairs there.
"I was walking down the street and I heard 'pop-pop-pop-pop-pop, ' " Janko said.
The city saw2,785 shooting incidents affecting almost 3,500 victims in 2017, according to police statistics, along with 650 murders. "I have never been thanked more for my service in the last two to three years total, compared to the previous 28 years".
Johnson sent out an email to members of his department, according to the Chicago Tribune.
"People were shouting at each other, because people thought it was a riot, and so did we", said Gloria Schmidt, who said she had been inside a nearby courthouse and came outside shortly after the shooting.
He is survived by his wife and 13-year-old daughter. "They taped it off, so we couldn't get out".
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He succeeds Steve Ells, founder and CEO of the Denver-based Chipotle , who announced in November he would step down. Chipotle has faced declines sales and traffic since several food-borne illness outbreaks in 2015 and 2016.