An explosion at a post office in Indiana on Wednesday left one person injured and is being investigated by the F.B.I.

The blast from a pipe bomb struck at the facility in East Chicago shortly after 6:30 p.m. Central time. One postal employee was treated for minor injuries, an F.B.I. spokeswoman, Chris Bavender, said.

“At this time, investigators are still trying to ascertain if there are any terrorism links,” Ms. Bavender said Thursday.

The city’s fire chief, Anthony Serna, said in a news conference Wednesday night that investigators believed the blast had come from a package. Two people were in the building and one was outside when the explosion happened, he said. The building had already closed for the day.