In a news conference at the scene of the tragedy, theories about what may have caused it were put to the police.
Some Belgian and Swiss media have run reports that the driver was occupied inserting a DVD just before the crash, claiming that is what some surviving children had told their parents.
The local prosecutor has cast doubt on this and
police have said it is pure speculation.
“We are not ruling out any leads,” said Christian Varone, the police chief heading the enquiry. “We are examining them all and not giving priority to any of them.”
Both the drivers were killed.
A total of 52 people were on the bus, carrying school groups from the Belgian towns of Lommel and Heverlee when it crashed into the wall of a tunnel.
Families who travelled to Switzerland have now identified all those killed, most of them around 12 years old. Their bodies are due to be flown home today.
Of the survivors, three children remain in a critical condition. Several others with minor injuries have been able to head back to Belgium.