LA PAZ, April 29 (Xinhua) -- At least nine people were killed and 22 others injured Wednesday when a passenger bus overturned on a highway in western Bolivia, police said.

The accident occurred early in the morning near the town of Villa Challacollo on the Pisiga-Oruro international route, which connects Bolivia and Chile.

Police said the driver, a 24-year-old man, tested negative for alcohol and sustained a moderate brain injury.

Authorities said possible causes include speeding and driver fatigue, though investigations are ongoing.

Rescue teams have transported the injured to hospitals for treatment. ■