HELPER — A chemical spill shut down a stretch of U.S. Highway 191 near Helper Tuesday.

Officials are still investigating what caused the driver of a semitrailer truck to lose control, rolling the vehicle at about 10:30 a.m., said trooper Cameron Roden of the Utah Highway Patrol.

The vehicle got stuck on a guardrail, which punctured the gas tank, spilling more than 100 gallons of diesel fuel onto the road. Two or three gallons of the truck's cargo, ammonium nitrate, spilled onto the road, Roden said.

"They're taking things real slow," he said of the cleanup efforts. "They have been concerned with the combination of the diesel fuel and the nitrate. It is very explosive."

The highway was still closed in both directions at about 6:30 p.m. Roden said he expected crews to finish cleanup and open the road late Tuesday night.