The Belgian authorities are preparing to fly home the bodies of 22 children and six adults killed in a coach crash in Switzerland.
Two C-130 Hercules military planes are on standby. The Belgian cabinet is due to hold an urgent meeting shortly.
The coach, carrying 52 people back to Belgium following a skiing trip, struck a wall head-on in a tunnel on Tuesday.
Relatives of the victims were later flown to Switzerland, many still unsure about the fate of their children.
Another 24 children were injured, some critically, in the late night accident near the town of Sierre, in the southern Swiss canton of Valais.
It was the most serious traffic accident in Switzerland's history for decades.
The Belgian foreign ministry said most of the children were aged about 12, and the bus was one of three hired by a Christian group. The other two reached Belgium safely.
The children had spent a week skiing in Val d'Anniviers in the Swiss Alps.
Those on board the bus that crashed were from the Stekske primary school in Lommel, near the Dutch border, and from St Lambertus in Heverlee, near Leuven (Louvain).
Although most of the victims are Belgian, Swiss officials say the dead include seven Dutch children. Among those injured are three Dutch, one Pole and one German.
Early on Wednesday, distraught relatives attended meetings at St Lambertus school, where they were given the names of pupils known to have survived.
"Parents who know their child is alive are relieved, but for the others it's terrible," said parish priest Dirk De Gendt.
Speaking outside the primary school in Lommel, local bishop Patrick Hoogmartens said the families were feeling powerless.
"None of the parents knows what has happened exactly, if their child has been affected or not," he said.
About 100 relatives later flew to Switzerland on board a government plane.
Prime Minister Elio Di Rupo - who also travelled to Switzerland on Wednesday - told reporters in Sion that it was "an absolutely tragic day" for Belgium.
At a news conference on Wednesday, Swiss prosecutor Olivier Elsig said the bus was nearly new.
It hit a concrete wall that forms part of an emergency access section head-on, in a tunnel where the speed limit is 100km/h (62 mph).
Mr Elsig said the speed of the bus was still being determined, but there was no indication that it was travelling too fast.
The children on the bus were wearing seat belts and no other vehicle was involved, he added.
Mr Elsig said that possible causes being investigated included a technical failure, the driver suffering a health problem, or "human error".
Meanwhile, Belgian Transport Minister Melchior Wathelet said the company that ran the coach, Toptours, had "an excellent reputation".
He said: "The drivers had arrived the night before and had rested during the day before departure. It seems that the law on driving and rest periods has been respected."
Both the drivers were among the dead.
Valais police chief Christian Varone described the site as "a scene like a war".
On Wednesday, Belgium held a day of national mourning, while the Swiss and the European parliaments observed a minute's silence for the victims.
US President Barack Obama offered his deepest condolences to the families of the victims, saying that "the United States stands ready to provide whatever assistance may be helpful".