An explosive device detonated at the East Chicago Post Office and injured one worker on Wednesday evening, the Chicago Tribune reported.
Firefighters were called to the post office shortly before 6 p.m. following reports of an explosion, East Chicago Fire Chief Anthony Serna told the Tribune.
A woman was brought to St. Catherine's Hospital in East Chicago with non life-threatening injuries. At an evening press conference, Serna said that he couldn't confirm whether additional explosive devices had been found because firefighters did not enter the building.
"Bomb threats, yes," Serna said. "An actual explosion like this is rare."
The Federal Bureau of Investigation has now taken charge of the case.