Ten dead, including five students, as truck crosses motorway and slams into bus

Investigators do not yet know what caused a tractor-trailer to veer across a motorway reservation and slam into a bus, killing 10 people, the California Highway Patrol has said.

They are looking into various factors, including whether the FedEx tractor-trailer driver fell asleep, experienced mechanical failure or lost control because of a separate collision on the southbound side of the road.