A Belgian tour bus slammed head on into a concrete wall inside a Swiss Alps tunnel, killing 22 children and six others.

The accident in the short Tunnel de Geronde left 24 other children injured.

The tragedy occurred in the southern town of Sierre around at nine p.m. Tuesday, local time.

The bus was one of three buses that were rented by two Belgian elementary schools and were returning to Belgium from a weeklong ski trip in Switzerland.

Swiss police are investigating if the accident was due to mechanical failure or human error.