CHICAGO (CBS) — Federal authorities have made an arrest in the case of a pipe bomb that exploded at a post office in East Chicago on Sept. 6th.

Police say the bomber, 45-year-old Eric Krieg of Munster, Ind., was a mechanical engineer and supervisor at BP in Whiting.

Krieg was charged with possession of a destructive device and transporting explosive materials. If he’s convicted, Krieg could be sent to prison for up to 20 years.

Federal prosecutors say last month he dropped off a bomb at the post office in East Chicago. The explosion injured a woman, who was working in the facility, and was seven months pregnant at the time. She is reportedly doing well considering what happened.

That explosive device was meant to be delivered to an attorney working for the city of Hammond. Then in late Sept., prosecutors say Krieg mailed a letter that was found at the post office addressed to another Hammond city employee.

That letter contained a bullet along with written threats, according to prosecutors.

The mayor of Hammond says he gave authorities Krieg’s name immediately after the package bomb blew up in East Chicago.

“When the second target received the package, there was a link between the two that was unmistakable,” mayor Thomas McDermott, Jr. said.

McDermott describes Krieg as an extremist, saying, “We knew they were aiming at us — the workers of the city of Hammond. The guy that’s guilty, in my opinion, he was the first guy that jumped into my mind. Somebody that’s smart and weird enough to do something like this.”

U.S. attorney Tom Kirsch said they have zeroed in on a motive, but did not want to elaborate on it as of yet.

Krieg is married with four children. He was active politically, and ran for municipal office as a Republican. Krieg ran for Lake County Council then Lake County Surveyor. He lost both races.

Krieg once accused McDermott of misusing campaign funds, an allegation that the county election officials concluded to be unfounded.

Federal investigators served search warrants at Krieg’s home and work and he was arrested on Thursday.