Cause of deadly bus crash unknown SION, Switzerland, March 15 (UPI) -- Tunnel safety and rules for taking school-related trips were discussed by Belgian and Swiss media after a bus crash killed 28 people, including 22 students. Still unknown is what caused the bus to hit the edge of the road then crash head-on into a concrete wall in the Sierre tunnel in Switzerland Tuesday night, killing 22 children and six adults, and injuring 24 others, Expatica reported Thursday. Officials said the bus had entered the tunnel at about 62 mph when it hit the edge of the roadway then struck the concrete wall that served as an emergency stop. Tages-Anzeiger in Zurich noted angled safety recesses, such as the one into which the bus crashed, were positioned along the tunnel. Le Temps in Geneva quoted the Swiss transportation officials as saying the Sierre tunnel was built in 1999 and has up-to-date emergency exits, signaling and energy supply, and ventilation systems. The bus was returning the students, who were about 12 years old, and adults to two Belgian villages from a trip to Val d'Anniviers ski resort in Sion. Two Belgian military transport aircraft were made available to return survivors home as early as Thursday. Belgian Prime Minister Elio Di Rupo and other officials went to the crash site Wednesday. The ski trip was an annual event to mark the end of elementary school, parents told The New York Times. "The kids save up the money all by themselves for their last year, selling things, running bingo sessions and so on," said Vicki Emmers, a former student in Lommel-Kolonie. It's a dream from when you're just 5 years old to make this trip. Children from schools in Lommel-Kolonie and Heverlee were on the trip I'm asking myself, what are the drivers doing before they start out on their journeys? Jos Bode, whose 12-year-old granddaughter was on the bus, told the Times. Maybe it was not responsible of the tour operators to be driving at night. He said he did not know whether she had survived. The Times said radio and television across Belgium featured discussions with parents and drivers on whether the rules for bus trips were sufficiently rigorous.