LA PAZ, June 15 (Xinhua) -- At least 12 people were killed on Friday when a passenger bus crashed along a rural highway in southwest Bolivia, local police said.

Another 30 people were injured in the accident aboard a bus traveling between Cochabamba and Potosi, police in the town of Potosi said.

Police commander Col. Vladimir Lazo said the injured were taken to local hospitals, while rescue workers retrieved bodies at the scene.

"We are carrying out an investigation to determine the causes of the accident, whether it was recklessness or mechanical failure," said Lazo.

A preliminary police report said a speeding bus crashed into a boulder on the highway near Potosi.