A bus carrying a Belgian school party home from a ski trip crashed into the wall of a tunnel in Switzerland late on Tuesday, killing 28 people, mostly children.
Distraught parents, many of whom still did not know if their children were among the 22 pupils killed, gathered at a school in Belgium to be flown to Switzerland on military aircraft.
"Some parents know their kids have survived, but for others there is no news," said Belgian police spokesman Marc Vranckx.
The bus, transporting 52 people, mostly children aged about 12 from the towns of Lommel and Heverlee in Belgium's Dutch-speaking Flanders region, crashed in the Swiss canton of Valais at 9:15 p.m.(2015 GMT).
A police photograph showed the bus rammed up against the side of a tunnel, the front ripped open, broken glass and debris strewn on the road and rescue workers climbing in through side windows.
Children at St Lambertus school in Heverlee, a suburb of Leuven, were informed about the accident at an assembly before classes. Flowers were laid outside the Catholic school where eight children were still unaccounted for.
"The eight sets of parents, they can only sit and wait, they just don't know. I'm in pain, I have tears inside," Dirk De Gendt, a local priest who is on the school board, told Reuters.
"We don't have words, only deep grief. They were supposed to be back now."
A teacher and an assistant from St Lambertus were killed along with the bus's two drivers and two other adults. Twenty-four children were being treated in hospital for injuries.
Police said the bus had just joined the highway towards the Swiss town of Sierre after coming down from the resort. After travelling 2 km (1.2 miles) on the road, the bus bumped into the curb and skidded into an emergency siding in the tunnel.
The front third of the bus was completely torn apart. Many children were trapped in the wreck and had to be freed, said police.
About 200 police, firefighters, doctors and medics worked through the night, while 12 ambulances and eight helicopters took the injured to hospital.
"It is a sad day for all of Belgium," Belgian Prime Minister Elio Di Rupo said in a statement expressing his "great horror". He said he would travel to Switzerland on Wednesday.
Swiss parliamentarians stood for a minute's silence and President Eveline Widmer-Schlumpf said Switzerland would do all it could to support the injured and their families as well as the relatives of the dead.
Swiss prosecutors are investigating whether the cause of the accident was a technical problem or human error. The speed limit in the tunnel was 100 km per hour.
The bus owned by Belgian company Toptours was returning to Belgium from a skiing holiday camp in Val d'Anniviers, a resort in the Valais Alps that border France.
Belgian transport minister Melchior Wathlelet said Toptours had a solid reputation and the driver had arrived at the resort a day before the trip, according to the rules, and that the bus had passed a mechanic's test five months earlier.
"It is most important to clarify and then to understand what actually happened. If necessary we will have to take measures. Safety on the roads is my main priority," he said in a statement.
Switzerland's mountain regions have a history of deadly crashes. Tuesday's was the worst since 1982 when 39 German tourists were killed on a railway crossing when a train hit their bus. Twelve people were killed and 15 injured when a bus crashed into a ravine in the Valais region in 2005.
"We have seen a number of catastrophes in Valais but we have never experienced such a large number of young deaths. Wea are really totally upset," Jean-Pierre Deslarzes, head of the rescue operation, told Swiss television.
Last month, a British teacher was killed and more than 20 people hurt in northern France after a coach crashed while bringing school children home from a skiing trip in Italy.
Ten Dutch children were on the bus, nine who live in Belgium and one in the Netherlands, a Dutch foreign affairs spokesman said. But he could not comment on their condition, saying that the Dutch ambassador was at the scene.
The trip was coordinated by Intersoc, a Belgian social organisation which organises trips for members of Christian social and health funds. Two other buses on the tour returned to Belgium safely.