Updated

Chinese police say a disgruntled driver started a fire aboard his school bus last month that killed 13 people, including 11 children from China and South Korea.

A statement from the city government of Weihai on Friday said the driver in the May 9 fire had apparently been angered by the halting of his overtime bonus and night shift pay.

He bought gasoline which he ignited while the bus was travelling through a tunnel in the coastal city that is home to many South Korea businesses. Police determined that the fire started on the floor of the bus next to the driver's seat, where the cap to a cigarette lighter and gasoline residue were found, the statement said.

Investigators ruled out a traffic accident or electrical short circuit as potential causes of the fire, the statement said.

All 13 people aboard the bus were killed, including the driver himself, a female teacher and 11 children between the ages of 3 and 6, five of them from South Korea.