GENEVA -- Swiss authorities say they are narrowing their focus to the bus driver in a probe of a March 13 crash that killed 28 people, most of them children.

Twenty-two students returning from a ski holiday and six adults died when the bus crashed inside a tunnel in southern Switzerland on its journey back to Belgium. The 32-year-old driver was among the dead.

Prosecutors in the canton (state) of Valais said Friday their investigation has excluded the possibility another vehicle was involved, or that the road or tunnel were defective.

They also excluded speeding, alcohol or a technical problem with the bus as possible causes.

Chief prosecutor Olivier Elsig said in a statement that "from now the investigation is concentrating on causes linked to the driver of the bus."