Swiss investigators have ruled out excessive speed and alcohol as the cause of the school bus crash.
They are probing other possibilites, including human error and a technical problem with the coach.
The local police chief told reporters that officers had checked the coach’s tachograph – a system which records speed.
They found the driver was doing less than the permitted speed limit when the vehicle crashed inside a tunnel.
They also found no traces of alcohol in his blood.
Some media reports have suggested he was trying to change a DVD and became distracted, causing him to crash as the bus returned from a school ski trip in the Swiss Alps.
But that has been denied by the bus operator and dismissed as pure speculation by police.