Authorities say at least four people were killed and several more suffered serious injuries when a passenger bus overturned on Interstate 10 in Southern California near the Arizona border.

Caltrans spokeswoman Terri Kasinga says the crash occurred shortly after 2 a.m. Wednesday just east of Blythe when a big rig jackknifed and spilled a load of steel pipes into lanes.

She said the bus either swerved to avoid the pipes or it struck the pipes and overturned onto the shoulder.

Kasinga says four people are dead but there could be more fatalities.

Emergency airships have transported several passengers with critical injuries to hospitals.

Kasinga says all lanes of the interstate will be closed for several hours.