A disgruntled power plant employee harassed and threatened union officials before police discovered his carload of explosive materials and bullets, according to charges.

Robert James Johnson, 58, was charged last week with three felony counts of terroristic threats.

According to a criminal complaint, St. Paul police were contacted by the North Central States Regional Council of Carpenters in St. Paul regarding Johnson’s escalating behavior. Johnson had been confrontational with union representatives leading up to his May 13 firing from the Monticello nuclear power plant and became increasingly threatening in the week that followed, union officials told police.

Johnson told one union official, “You’d better resign or the last thing you’re going to hear …” and then Johnson “made a whistling sound like a bullet,” the complaint said.

Johnson told the official that he planned to be at a May 20 union meeting, that he was an ex-Marine and that he had a rifle with a tactical scope, the complaint said.

On May 20, officers arrested Johnson and towed his vehicle. A search of Johnson’s Cadillac produced leg irons, a container of lighter fluid, a container of propane, 6 pounds of the explosive Tannerite, 120 batteries, 500 rounds of .22-caliber long rifle shells, nine 20-gauge shotgun shells and a 5-gallon bucket, the complaint said.

Asked what he was planning to do, Johnson told investigators, “A surprise attack,” the complaint said. He told investigators he had a bear trap and ammunition in his car and then clapped his hands and said, “Boom. Tick tock tick tock,” the complaint said.

A bomb squad checked the union hall grounds for explosives but found nothing, according to the complaint.

The complaint said Johnson worked at the Monticello plant for three months and “has a history of being removed or fired from job sites for a variety of reasons.” His address is listed as Sioux Falls, S.D.

Johnson posted bond and was released from jail Tuesday. His attorney did not immediately return a call for comment.

Elizabeth Mohr can be reached at 651-228-5162. Follow her at twitter.com/LizMohr.