Fire broke out on the roof of the city's vacant former post office Tuesday where a movie crew had been filming scenes for an upcoming Batman sequel.
Fire department spokesman Kevin MacGregor said a building engineer suggested the fire might have started in the ventilation system. He didn't know of any injuries.
At least a dozen ambulances and fire trucks were at the scene, just south of Chicago's downtown business loop.
MacGregor said it was unclear if the fire was related to the filming.
The crews had been filming there all week, said Pete Kearney, who works next door. Letters affixed to the side of the old post office read: "Gotham National Bank."
"We assumed it was part of the movie," Kearney said of the smoke Tuesday morning.