The Morton County Sheriff's Department said a plane did not drop gas on people Wednesday during a protest at construction sites near St. Anthony.
Spokesman Rob Keller said the department investigated a spray plane, which was alleged to have dropped mustard gas on protesters.
"That’s a rumor," Keller said. “It’s not mustard gas.”
Keller said it was an agricultural spray plane that had been contracted by a farmer to spray a field.
On Wednesday, the plane had made several trips from the Mandan Airport to various sites in Morton County, he said. On its way back "as a safety protocol," it dumped vegetable oil into its exhaust, which created white smoke.
The smoke was used to make the plane visible to other aircraft in the area, Keller said.
The smoke was "the same kind seen routinely at air shows," he said.
“It’s foolish to think we would drop mustard gas," he said.
The North Dakota Highway Patrol said the plane was flying 400 to 500 feet above the ground, about a half-mile from demonstrators.
Twenty-one protesters were arrested Wednesday on charges including resisting arrest, criminal trespass on private property and possession of stolen property.