An Illinois man on trial in the bombing of a downtown Salt Lake City library was seeking revenge for the arrest of his son on a driving under the influence charge, federal prospectors claimed Thursday.

Thomas Zajac, 57, faces a six-count indictment for the Sept. 15, 2006, explosion that knocked out a library window and forced 400 people to flee. No one was injured.

In opening statements, Assistant U.S. Attorney for Utah Eric Benson said Zajac's anger over the 2004 arrest of Adam Zajac by Salt Lake City police led him to build a bomb, drive 1,500 miles to Utah and ignite the device on the library's third floor. He later claimed responsibility in an anonymous letter to Salt Lake City police that threatened other bombings, Benson said.

"All because of his son ... He had been plotting his revenge against (police) all these years," Benson told the U.S. District Court jury.

Thomas Zajac was arrested a month after the bombing at his home in Downers Grove, Ill. Investigators said they found his fingerprint on a price tag from a Hobby Lobby store on a piece of cardboard to which the bomb was attached.

Defense attorney Deirdre Gorman told jurors Zajac has been wrongfully accused.

"What the government has described is way out of proportion with what really happened," Gorman said. "No one was killed. The window cracked and the frame dented."

Gorman said no library security cameras recorded the bombing and witness accounts varied widely about the details of what occurred and in descriptions of possible suspects.

In court papers, authorities say the bomb was made with an igniter available at hobby shops, along with galvanized pipe, a 9-volt battery and a 60-minute kitchen timer. It was stuffed inside two white paper bags from a fast-foot restaurant.

An indictment charges Zajac with one count of carrying a destructive device in relation to a crime of violence, which carries a 30-year mandatory sentence that must be served consecutive to any other sentence. Five other charges carry sentences of five to 10 years.

Zajac pleaded not guilty in November 2006. Two previous trial dates were set, but the case dragged on as he changed attorneys at least five times.

In May, a federal grand jury in Illinois issued a four-court indictment alleging Zajac exploded a pipe bomb at a suburban commuter train station in 2006 in Hinsdale.

On Thursday, Benson said that bombing occurred exactly two weeks before the Utah blast and was followed by an anonymous letter to Hinsdale police. As with the Utah case, Benson said the incident was motivated by a run-in between Adam Zajac and Hinsdale police.

A hearing in the Illinois case is set for Oct 21.