SIERRE, Switzerland — A tour bus carrying schoolchildren home from a class trip slammed head-on into a tunnel wall in the Swiss Alps, killing 22 Belgian students and six adults and instantly changing a joyous skiing vacation into a tragedy spanning two European nations.

As authorities tried Wednesday to piece together what happened, parents, classmates and rescue workers struggled to grasp the awful turn of events. Only days earlier, the children had updated a lively blog about the highlights of their adventure: ravioli and meatball dinners, cable-car rides and sing-a-longs.

Police said the bus was not speeding and everyone aboard had been wearing seat belts when it crashed late Tuesday inside the 1.5-mile Tunnel de Geronde on a highway near the southern town of Sierre, a gateway to the Val d’Anniviers tourist region. No other vehicles were involved.

Belgian authorities flew anxious parents and relatives to the site and called for a day of mourning. The Swiss parliament held a minute of silence for the victims.

Investigators were still trying to determine how a modern bus, a rested driver and a seemingly safe tunnel could produce one of the deadliest highway crashes in Swiss history.

Olivier Elsig, prosecutor for the Swiss state of Valais, said officials were looking at three possible causes – a technical problem with the bus, a health problem with the driver or human error. He said an autopsy would be performed on the driver. “We will examine everything to find out what happened,” Elsig said.

Dr. Jean-Pierre Deslarzes, medical director of the local Swiss rescue service OCVS, said first-responders were traumatized because so many of the victims were children around 12 years old.

The accident virtually shredded the front end of the bus, leaving only small, barely recognizable pieces in place. Passengers were trapped inside.

“We found an apocalyptic situation when we arrived,” said Christian Varone, police commander for Valais.

Swiss President Eveline Widmer-Schlumpf also flew to Sion to pay her respects to the victims, survivors and rescue officials. A crisis center was set up for families of the dead.

Police said 21 of those killed were Belgian and seven were Dutch. Twenty-four other children were hurt.

The crash occurred in a stretch of tunnel where the speed limit was 62 mph. The bus veered, hit a curb, then rammed into a concrete wall in an emergency stop space, police said.

The bus was operated by the Top Tours company, based in Aarschot, about 25 miles northwest of Brussels. A woman who answered the phone at the company’s offices declined to comment, but Belgian Transport Minister Melchior Wathelet said Top Tours has a good safety record.