ST. PAUL — A disgruntled contract employee who had been working at Xcel Energy’s Monticello nuclear power plant harassed and threatened union officials before police discovered he had a carload of explosive materials and ammunition, prosecutors allege in a criminal complaint.

Robert James Johnson, 58, of Sioux Falls, South Dakota, was charged in Ramsey County last week with three felony counts of making terroristic threats. The complaint alleges he had been confrontational with carpenters’ union officials leading up to his May 13 firing and became increasingly threatening to them before his arrest May 20, as first reported by the St. Paul Pioneer Press.

The complaint said Johnson worked at Monticello for three months and “has a history of being removed or fired from job sites for a variety of reasons.” It said the steward at the job site told union officials May 10 he could no longer work with Johnson because of Johnson’s behavior. It said an attempt by a chapter president to mediate ended with Johnson being escorted out of the union office.

Johnson allegedly told another union official May 18 he was “an ex-Marine and certifiably crazy. … You’d better resign or the last thing you’re going to hear,” then “made a whistling sound like a bullet.”