No one was injured when the bomb exploded and damaged a window on the library's third floor the afternoon of Sept. 15, 2006. But prosecutors claimed the bomb was capable of killing.
Investigators tied Zajac to the explosion through a fingerprint on a scrap of paper found at the scene. The paper came from packaging for a toy rocket motor.
Zajac was placed in Salt Lake City that day through phone and credit card records. He was also identified on library surveillance video.
Assistant U.S. District Attorney Richard McKelvie told jurors that the Salt Lake City bombing was similar to an explosion in Hinsdale, Ill., two weeks earlier.
The motive for the bombings, McKelvie said, was "resentment" and "retribution" for perceived wrongs by police in those jurisdictions in which Zajac's son, Adam Zajac, was the victim.
Adam Zajac had been arrested by Salt Lake City police and charged with driving under the influence of a prescription drug immediately after a medical procedure. In Hinsdale, police arrested the son on an outstanding $100 warrant.
Anonymous letters written by the same person and sent to police chiefs in both jurisdictions complained of unfair treatment by police, and the letters were similar in style and wording, prosecutors said.
But defense attorney Ed Wall told jurors that Adam Zajac was a better fit for the bombing, since he was the one police had harassed and humiliated. Wall added that it is Adam Zajac's nature to hold things in and later lash out, while his father immediately expresses his feelings.
But McKelvie scoffed at the notion of a frame-up, noting that Adam Zajac would have had to get his father's fingerprint on a rocket motor wrapper, follow him to Salt Lake City, wait for him to enter a public place and then trust that the paper with the fingerprint would survive the explosion.
Zajac was charged with seven felony counts, but one of them was dismissed Monday because of a typographical error on the jury's verdict form. Prosecutors said dismissal of the count will make no difference at sentencing.