GENEVA (AP) — A bus smashed into a tunnel wall in the Swiss Alps, killing 22 Belgian 12-year-olds returning from a ski vacation and the six adults who were accompanying them, police said Wednesday.

Another 24 students were hospitalized with injuries, Valais cantonal (state) police spokesman Jean-Marie Bornet told The Associated Press.

The bus carrying 52 people, including young students from two different Belgian schools, hit the tunnel wall shortly after 9 p.m. local time Tuesday on the A9 highway near Sierre, Switzerland, in the southern Swiss canton of Valais, an area of popular ski resorts.

Authorities were still identifying all the victims, said Didier Reynders, the Belgian foreign minister. It was unclear exactly how the accident happened, he said.

“It is incomprehensible,” Reynders said.

Belgian Prime Minister Elio Di Rupo said this is “an extremely sad day for all of Belgium.” He was flying with the families of the victims to Switzerland, the government said.

Police said the students had spent the last few days at a ski camp and were on their way back to the Belgian provinces of Brabant, Flanders and Limbourg.

The bus veered and hit a curb, then rammed into a concrete wall in the tunnel, according to police. The front of the bus was heavily damaged and appeared to be wedged against the wall, blocking people from getting out.

“The bus hit the barrier stones on the right side of the road. It then hit the tunnel wall front-on in an emergency stop space,” police said in a statement.