Twenty-two 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.