Police say a bus has run off a foggy mountain road in Bolivia, killing at least 28 people.

Police Col. Placido Arroyo tells the radio station Fidels that the bus plunged about 300 feet down a ravine on Sunday. It was en route from Villazon near the Argentine border to the city of Tarija.

Arroyo says the cause of the accident is under investigation, but he says there was a dense fog at the time of the crash.

It's Bolivia's worst bus accident of the year. Twenty-five people died in January when a bus and a truck carrying passengers tried to cross a flooded road.