The prosecutor for the Swiss canton Valais, Oliver Elsig, said investigators were looking into a couple of possible causes: a technical defect or human error.
"We will examine everything to find out what happened," Elsig said. "The tour bus was new and well maintained and according to the information we have, the driver was well rested."
The Belgian-registered bus was on its way back to Belgium from Switzerland when it crashed into the wall of a tunnel, killing 28 people and wounding the 24 other passengers.
An autopsy is to be performed on the driver to determine whether a sudden illness could have caused him to lose control of the bus.
The bus, carrying mostly Belgian and Dutch schoolchildren but also one German, crashed in the Geronde Tunnel on the A9 motorway near the town of Sierre late on Tuesday. It happened only about half an hour after setting off from the Val d'Anniviers region, where the schoolchildren had spent the previous few days on a skiing vacation. Belgian officials said the children were 11 or 12 years told.
Repatriating the bodies
Belgian authorities were preparing to fly home the bodies of the 28 victims, including 22 children, on Thursday. Around 100 relatives had flown to Switzerland on a government plane the day before.
Belgiumdeclared a day of morning, while both the European and Swiss parliaments observed minute of silence.
German Chancellor Angela Merkel was among several European politicians who expressed their condolences.
"I wish to express my personal sympathy to you and your fellow countrymen," Merkel said in a letter to Belgian Prime Minister Elio Di Rupo.
pfd/ncy (AP, dpa)