WASHINGTON -- The tobacco farmer whose protest on the National Mall caused massive gridlock in the capital last March was convicted Friday of two federal felony charges.

Dwight Watson, 50, of Whitakers, N.C., was convicted of making a false threat to detonate explosives and of destroying federal property. Each count could carry a prison sentence of as long as 10 years.

The jury deliberated barely one hour before reaching its decision.

The jury found that Watson had inflicted more than $1,000 in damage when he drove across Constitution Gardens and dug up part of a small island in a shallow pond between the Washington Monument and the Lincoln Memorial.

U.S. District Judge Thomas Penfield Jackson set a presentence hearing for Dec. 9.

Watson, who sat expressionless as the verdicts were read, had testified that he had merely engaged in civil disobedience and that he had told police he had an "organophosphate bomb" in a box. For two days, Watson held police at bay with what turned out to be a pair of aerosol cans of Raid insecticide he had purchased at a store in Rocky Mount, N.C., before making the 10-hour journey to Washington.

"Organophosphates are not explosives, cans of Raid are not explosives," assistant federal public defender Erica Hashimoto argued in court.

"Cans of Raid are bug bombs."

But Assistant U.S. Attorney Jay Bratt told jurors in closing arguments, "It is the practice of bomb technicians everywhere to treat the statement 'I have a bomb' as though it is serious."