(CBS) — A post office employee in East Chicago, Indiana, was injured in an apparent pipe bomb explosion Wednesday evening.

Federal investigators were working to determine who was responsible for the blast, which happened shortly after 5:30 p.m. at the post office at 901 E. Chicago Av.

Sources said the explosive device was possibly a pipe bomb.

Local, state, and federal law enforcement agencies swept the building after the explosion to determine if there were any other bombs inside. As of Thursday morning, authorities have not reported finding any other explosives.

East Chicago Fire Department officials tried to calm a chaotic scene Wednesday night, but wouldn’t say much about the explosion.

“Right now, I can only verify that there was one person injured. So far it’s minor injuries,” Acting Chief Francisco Mendez said Wednesday night. “I do not want to give you false information, so I can just tell you that it’s under investigation, and we got everybody that we need right now to take care of the situation.”

Post office officials did not respond to requests for an update on the employee’s condition Thursday morning.

The FBI said it is too early to determine if the explosion was an act of terrorism, but said “there is no ongoing public safety concern.”

It was not clear if the post office would reopen on Thursday.