EAST CHICAGO, Ind. (CNN) -- A postal worker was hurt when a suspected pipe-bomb exploded inside of a post office Wednesday night.

Firefighters in East Chicago say there was at least one detonation.

There were two people inside the building at the time of the detonation while one person was outside.

The fire chief says one employee inside had minor injuries.

"Right now I can only verify that there was one person injured. So far it's minor injuries. It was an employee. Yes," said Acting Chief Francisco Mendez of the East Chicago Fire Department.

The FBI is investigating the explosion along with the Bureau of Alcohol, Tobacco, Firearms and Explosives.