Authorities have arrested a man in an explosion last month at the city's main library after reportedly finding his fingerprints on remnants of a rocket igniter.
Thomas James Zajac is charged with possessing an unregistered destructive device, according to court documents unsealed Monday. He was in custody Monday.
Authorities planned a news conference later in the day.
The Sept. 15 explosion blew a hole in a third-floor window and damaged a chair. There were no injuries, but 400 people were evacuated from the downtown library.
Zajac has lived in Salt Lake City and Provo. His fingerprints were on file because of arrests in Ohio and Illinois, said Michael Minichino, a federal firearms agent.
Authorities said a hobby shop-type rocket igniter was used in the explosion.
The six-story, 240,000-square-foot library, with a swooping architectural design and more exterior windows than walls, opened in February 2003. It has a collection of more than 500,000 books and other materials.