© Agence France-PresseWreckage ... 28 people died in the bus crash
The mystery over the bus crash that killed 22 children and six adults in a Swiss tunnel is deepening.

As the bodies of the victims were returned to Belgium amid national mourning on Friday, a Swiss prosecutor in the case ruled out the driver being drunk, speeding or being distracted by a DVD player.

Although investigators were now looking into a possible technical fault with the bus, a driving error, or a medical problem, prosecutor Olivier Elsig made clear that the driver did not suffer a heart attack and had rested sufficiently ahead of Tuesday's accident.

"Pending the final confirmation of the examination, we can say that alcohol was not involved," Elsig told reporters, citing autopsy results.

He said the man was driving below the speed limit when his vehicle hit a tunnel wall, killing 22 sixth-graders, four teachers and the two drivers on the bus who were returning to Belgium from a skiing trip in the Swiss Alps.

Reacting to media reports that the driver may have been changing DVDs before the accident, Elsig said investigators had interviewed around 10 surviving children and other witnesses.

But there had been "not a single child, not a single witness who until now could confirm this rumour in any way," he told reporters in Sion, a western Swiss town near the accident site.

© ReutersPaying tribute ... a girl holds a rose while waiting in a bus before visiting the scene of the accident.
The examination of the vehicle and the analysis of the traces that were collected at the crash site were now ongoing, Elsig said.

Hours earlier, 22 white caskets with the children's bodies and six brown wooden ones with the adult victims arrived at an airport near Brussels aboard two Belgian military planes.

At 11am (2200 AEDT) the country observed a minute's silence for the dead children from two schools as well as the adults who lost their lives.

Public transport halted, people in Brussels' busy Gare Centrale train station stood still, and policemen and firemen around the country gathered to pay their respects.

A minute's silence was observed in the Netherlands too, where six of the victims lived.

Many of the 24 children who were injured in the crash returned home, while four others were so severely injured that they could not travel, news agency Belga said.

The coach was carrying children aged 11 and 12 when it smashed into the wall of an emergency bay of a tunnel in the canton of Valais.

The children came from two schools in the Flemish part of Belgium, one in Lommel near the Dutch border and the other in Heverlee near Leuven.