Chicago police bomb technicians responded around 11:50 a.m. to the 4700 block of South Marshfield Avenue, in the Back of the Yards neighborhood, for a report of a suspicious object, police said.
Investigators discovered the object inside the apartment building and determined it to be an explosive.
No one was injured and part of the building was evacuated as police removed the object.
There is no current danger and the scene was secured, Chicago police said.
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No further information about the explosive device was immediately available.