Prosecutors say a disgruntled contract worker at a Minnesota nuclear power plant harassed and threatened union officials before police discovered he had a carload of explosive materials and ammunition.

Fifty-eight-year-old Robert James Johnson, of Sioux Falls, South Dakota, is charged with three felony counts of making terroristic threats. The St. Paul Pioneer Press reports (http://bit.ly/1J9C5F3 ) the complaint alleges he'd been confrontational with carpenters' union officials before his May 13 firing from the Monticello plant and became increasingly threatening before his arrest May 20.

It says a search of his car found 6 pounds of the explosive Tannerite, lighter fluid, propane, 500 rifle rounds, nine shotgun shells and leg irons. It alleges he told investigators he was planning a surprise attack.

His attorney didn't immediately return a call seeking comment Thursday.

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Information from: St. Paul Pioneer Press, http://www.twincities.com