The blaze started at 6:29 p.m. on the roof of the old post office building at 433 W. Van Buren St., according to Fire Media Affairs.
Fire crews surrounded the building and closed streets while inside firefighters searched for the source of the blaze.
"It was just a little bit of a logistical nightmare because of the size or footprint of the building, certainly getting instructions on how to get to the correct location," said Deputy Chief Tony Van Buskirk, Chicago Fire Department. "We could see it from the outside but getting to it from the inside was more difficult."
Authorities say no one was inside the building when the fire started and no one was injured. It took about one hour to strike the fire.
Onlookers in the Loop gathered nearby.
"I saw the smoke coming over the intersection. It was white smoke and I smelled something like rubber was burning," said Cody Miller, a firefighter from near Philadelphia visiting Chicago.
The last fire in the building in 2007 happened while the movie Batman: The Dark Knight was filming outside. The source of that fire was a burned-out motor in the ventilator room. Firefighters suspect that was the case in Friday's fire.
"Those combustibles started a small fire that got into the roof of that shaft. It was contained within that area, it took us just awhile. We controlled it with hand pumps," said Van Buskirk.
The Sun-Times Media Wire contributed to this report
