A blaze at the vacant Old Chicago Main Post Office building Tuesday morning in the Loop was likely caused by trains running under the structure.
Firefighters responded Tuesday morning to a fire at Chicago's Old Post Office.
Crews called an EMS Plan 1, which sends five ambulances to the scene of a high-rise fire.
A cloud of gray smoke was seen emanating from the building, at 433 W. Van Buren Street. Video from NBC Chicago's helicopter showed firefighters on a high-rise ladder.
Officials said the fire in the vacant building was under control by 9:30 a.m. without any injuries.
A vent which runs the full height of the building and empties from the roof caught fire, Fire Media Affairs Director Larry Langford said.
The material that coats the inside of the vents sometimes catches fire from idling or passing Metra trains, which run below the building, he added.
Built-up tar in a ventilation shaft of the building ignited by a passing or idling train caused a fire on the roof of the building in February 2012.
The Eisenhower Expressway -- Interstate 290 -- cuts through the building as it becomes Congress Parkway. The United States Postal Service vacated the building in 1997 for a newer facility.
