A tour bus that ran into a tunnel wall killing 28 people in the Swiss Alps was not speeding, a Swiss prosecutor revealed Wednesday.

The bus carrying 52 people hit the wall of the Tunnel de Geronde shortly after 9 p.m. on a highway near the southern town of Sierre, Switzerland.

The bus was full of students, mostly 12-year-olds, who were returning to Belgium after a skiing vacation. Twenty-two children and six adults were killed in the crash. Twenty-four other students remain hospitalized.

Olivier Elsig, prosecutor for the canton of Valais, also told reporters the children on the bus were wearing seat belts and no other vehicle was involved.

Investigators on the scene were looking at three possible causes for the crash: a technical problem with the bus, a health problem with the driver, or human error.

Local authorities are still working to identify all the victims.