A bus crashed into a tunnel Tuesday night in Switzerland, killing 28 people, including 22 children, police said.

Another 24 children were injured in the wreck, police said.

Most of the children were 11 and 12 years old, said Joren Vandeweyer, a spokesman for the Belgian Ministry of Foreign Affairs.

The bus was on its way back to Belgium from a ski trip. It slammed into a highway tunnel in Sierre in the Swiss canton of Valais.

The bus was carrying 52 people: two drivers, four other adults and 46 children.

The cause of the crash is under investigation.

The bus was returning from Anniviers in Switzerland when it crashed, said an online statement from police in Valais.

The children on the bus attended two schools in the Belgian towns of Heverlee and Lommel, it said.

Speaking to the press Wednesday morning, prosecutor Olivier Elsig assured the affected families that he would do everything possible to find out what had happened, according to the police statement. He said the bus was new or nearly new, and was fitted with seat belts.

The speed limit in the tunnel was 100 kilometers per hour (62 mph), the statement said.

Jean-Pierre Deslarzes, medical chief of the Canton Valais rescue organization, is quoted as saying the accident had upset and shocked even the most hardened of the rescue personnel.

There has never been an accident involving such a large number of children, he said, according to the police statement. Some 200 rescuers had worked for eight hours to help the injured, Deslarzes said.

Belgium has made available two military planes to take parents to see their injured children and to bring them back, Vandeweyer said.

"This is a tragic day for Belgium," Belgian Prime Minister Elio Di Rupo said, adding he will be traveling to Switzerland on Wednesday.