The Top Tours company, based in Aarschot, about 25 miles (40 kilometers) northwest of Brussels, was in charge of the bus that crashed. A woman who answered the phone at the company's offices declined to comment, but Belgian Transport Minister Melchior Wathelet said it has a good safety record.
"The company has an excellent reputation. The drivers had arrived (in Switzerland) the night before and rested on the day before the departure. It seems that the rules regarding driving and rest time were respected," said Wathelet. He added the bus that crashed was relatively new.
Two other buses, carrying students from schools in the Belgian towns of Beersel and Haasrode, arrived safely back in Belgium on Tuesday, apparently without having seen the accident.
Dutch foreign ministry said 10 of the children on the bus were Dutch, but said it was not clear if any of them were among the dead.
The previous worst accident in a Swiss highway tunnel happened in 2001, when two trucks collided in the Gotthard tunnel, killing 11 people.
One of Europe's most tragic tunnel accidents happened in March 1999, when 39 people were killed after a truck caught fire in the Mont Blanc tunnel between France and Italy. The blaze burned for two days while firefighters tried to reach victims and vehicles trapped in the tunnel under Western Europe's highest peak.
At midday, the blog of the Sint Lambertusschool in Heverlee was still online, showing kids smiling and frolicking amid the snow.
"Things are super here in Saint-Luc. The skiing, the weather, the food. Its all not bad at all," one boy posted on Saturday. "Tomorrow I play in the Muppet Show. ... I'm now reading the book 'Why Dogs Have Wet Noses.' Very interesting! I miss you all."
Melvin reported from Brussels. Frank Jordans in Geneva, Raf Casert in Heverlee, Belgium, and Robert Wielaard in Brussels contributed to this report.
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