CHICAGO (CBS) — The post office in East Chicago, Indiana, reopened on Thursday, after a postal worker was injured in a pipe bomb explosion a day earlier, but the FBI has yet to determine who is responsible.

The FBI confirmed the blast was the result of a single pipe bomb that went off shortly after 5:30 p.m. Wednesday, causing minor injuries to a postal employee.

READ MORE: Chicago Weather: Rain Followed By Colder Temps

Dozens of local, state, and federal law enforcement investigators descended on the building after the explosion, and searched the building to determine if there might be more than one bomb.

Thursday morning, the FBI said there was only one explosive device, but the motive for the bombing remains unclear.

“At this time, investigators are still trying to ascertain if there are any terrorism links,” the FBI said.

READ MORE: My Block, My Hood, My City Volunteers Robbed While Holiday Decorating In Bronzeville

Anyone with information about the explosion should call the FBI at 800-225-5324, or go to

The post office opened for business as normal Thursday morning. Customers who pick up their mail there said they were floored when they learned of the explosion.

“I was just shocked, because it was the first time I hear about something like this in our city, so it’s kind of alarming,” James Stowe said. “It’s something different. You know, you hear about bombs everywhere else, but a post office, not too much.”

A post office employee said the facility was operating as usual on Thursday. The lobby opened a few minutes after it was scheduled, around 6:15 a.m.

MORE NEWS: Teen, 15, Shot In The Loop

Mail carriers loaded up their trucks and shipped out shortly after 9 a.m.

Lauren Victory