(Reuters) - A pipe bomb exploded at a post office in northwest Indiana on Wednesday, injuring one worker, media reported.

The motive for the attack was not immediately clear and the FBI was investigating.

Firefighters responded at about 6 p.m. local time to the post office in East Chicago, about 25 miles (40 km) southeast of Chicago, where a female postal worker was hurt, the Chicago Tribune reported, citing East Chicago Fire Chief Anthony Serna.

The worker, who was not identified, was taken to a hospital where she was being treated for non-life-threatening injuries, the Tribune reported.

Two employees were inside the post office at the time of the explosion and another was outside, the newspaper reported.

Firefighters did not enter, but set up a perimeter around the building, Serna told the newspaper.