(CNN) -- 22 children and six adults died when a bus crashed into the wall of a tunnel in Switzerland, police said.
Another 24 children were injured in Tuesday night's wreck, authorities said.
Three children are hospitalized in critical condition, Jean-Pierre Deslarzes, medical chief of the Canton Valais rescue organization, told a news conference Wednesday afternoon. Others are expected to be discharged from the hospital in the coming days.
The bus was on its way back to Belgium after a ski trip when it slammed into the side of a highway tunnel in Sierre in the Swiss canton of Valais.
It was carrying 52 people: two drivers, four other adults and 46 children, aged 11 and 12 years old.
Prosecutor Olivier Elsig told reporters at the news conference that the cause of the crash was still under investigation, but initial inquiries indicated the bus, which was new, was not speeding and that the road conditions were good.
Police have said the speed limit in the tunnel was 100 kilometers per hour (62 mph).
Experts are studying CCTV images and speaking to witnesses and some of those on the bus who were less badly injured to find out what happened, Elsig said, adding that no other vehicle was involved in the crash.
An autopsy will be carried out on the bus driver, who is among the dead.
Investigators are focusing on three potential causes for the accident, he said -- a technical issue with the bus, a health problem with the driver or human error.
Rescuers reported that the children were wearing their seat belts, he said, but the impact of the crash was so great that the belts did not save their lives.
Christian Varone, police chief for Valais Canton, said investigators were still working to confirm the identities of the 28 people killed. All but two of those injured had been identified and their families contacted, he said.
"This is a tragic day for Belgium," Belgian Prime Minister Elio Di Rupo said.
He said there were 10 people of Dutch nationality among the victims as well as one of German and one of Polish origin.
No problems had been reported with the bus, and it had passed technical checks in October, Di Rupo said. The drivers had respected regulations regarding rest time, he added.
Pierre-Martin Moulin, assistant to the chief of police in Valais, earlier said authorities would need time to understand what had led the bus to hit the curb on the right side of the tunnel, then crash headlong into the wall of an emergency stopping point.
It was too early to speculate about the cause of the accident, he told CNN from Sion, Switzerland. The tunnel was built recently and had many safety features, he added.
The bus was returning from Anniviers in Switzerland when it crashed, said an online statement from the police in Valais.
The children on the bus attended two schools in the Belgian towns of Heverlee and Lommel, it said.
Deslarzes said the rescue operation had been "very painful" for the emergency workers because of the number of children involved.
"It was a difficult night for all the people engaged on the scene," Moulin said, expressing his condolences to families.
He said the rescuers, who numbered more than 200, had gotten quickly to the crash site but nothing could be done for the many children who were killed.
"To end a ski trip like that, there's no words to describe what happened there," he said.
Belgium has made available two military planes to take parents to see their injured children and to bring them back, Vandeweyer said.
The Swiss Assembly held a minute's silence Wednesday morning for the victims.
The European Parliament also fell silent to mark the tragedy. Martin Schulz, the assembly's president, paid tribute to the rescue workers and expressed his condolences to the families.
The local government in Valais also paid tribute to those killed and their families.
"That a holiday trip can turn into such a tragedy is met by the people of the Canton Valais with deep sympathy and mourning," a statement on its website said.
CNN's Laura Smith-Spark, Stefan Simons and Samira Said contributed to this report.