Helder faces federal charges in Illinois, Nebraska and Iowa, where his initial court appearance is scheduled for Friday.

If convicted, he could be sent to prison for life. Cameron Helder said he expects the legal proceedings to be a long process.

It's already been very hard on us," he said. "Our heart goes out to the families of the victims."

The FBI said Helder placed 18 pipe bombs in mailboxes in Iowa, Illinois, Nebraska, Colorado and Texas, along with anti-government notes. Six of the bombs exploded last Friday, injuring four letter carriers and two residents.

The eight bombs in Iowa and Illinois were found in rural locations that form an uneven ring about 70 miles in diameter. The Nebraska bomb sites - about 350 miles away - form a large ring about 90 miles across.

The other bombs were found hundreds of miles to the southwest - one in Salida, Colo., and another in Amarillo, Texas.

The FBI issued an alert for Helder after his father called police Monday night about letters from his son that included references to death, anti-government comments and the phrase "Mailboxes are exploding." The same phrase was in the notes found with the bombs.