Olivier Maire, Pool, Associated Press
Police officers carry the coffin of one of the 28 victims of Sierre's bus crash into a Belgian military cargo aircraft at Sion airport, western Switzerland, Friday, March 16, 2012. Twenty-eight people, including 22 children, returning to Belgium from a skiing holiday died in a bus accident inside a tunnel in Sierre in the Swiss canton of Valais.

BRUSSELS — Belgium held a national day of mourning Friday for the 28 victims of a school bus crash in Switzerland, ringing church bells and stopping trains and factories to show solidarity with heartbroken parents.

Swiss authorities, meanwhile, investigated whether the design of the tunnel contributed to the disaster.

Solemn music filled the airwaves in Belgium, and official buildings dropped their flags to half staff. At 11 a.m., the nation of 11 million held a minute of silence to mark Tuesday's crash, which killed 22 children returning from "snow classes" in the Alps — a traditional rite of passage in Belgium from childhood to the teenage years. Six adults on the bus — teachers, drivers and ski monitors — also died, and 24 students were injured in the crash.

Three days of national shock turned into heart-wrenching sorrow. Trains and subways stopped in their tracks for the tribute, political leaders stood united in silence, factory workers dropped their tools and Belgian churches slowly chimed bells in unison.

Around noon, a long line of black hearses left a Brussels military airport to take the victims back to their hometowns.

Flags were lowered over the Belgian parliament and in the Netherlands, too. Six Dutch kids, who attended school just across the border in Belgium, were among the dead. One of the dead students was a Briton living in Belgium, said the St. Lambertus School in Heverlee, Belgium.

"You hand over your child for a school trip and you always have the fear about a safe return," said Dutch Prime Minister Mark Rutte. "And in this case, we have this terrible outcome."

"We share this sorrow," Rutte said. "We have victims, and it totally doesn't matter what passport the children or their parents have."

The tourist bus carrying 52 people crashed head-on into a wall inside a tunnel as it headed home from a ski vacation in the Swiss Alps.

Olivier Elsig, prosecutor for the Swiss canton of Valais, said the crash is being investigated for three possible causes — a technical problem with the bus, a health problem with the driver or human error. Investigators have determined it was a modern bus with two rested drivers, that it was traveling within the 100 kilometer-per-hour (62 mile-per-hour) speed limit, and that the tunnel was considered safe.

But the Switzerland Federal Office for Roads said Friday it was examining whether the angle of the wall that the bus hit contributed to the severity of the crash. That part of the tunnel had a cutout for disabled vehicles, which meant part of the wall was at a right angle to the tunnel road.

"In principle there is the possibility of slanting the angle of the bay, or protecting it with concrete or other elements," spokesman Michael Mueller told The Associated Press.

But he cautioned that modifying the design of tunnel safety bays to better protect buses could have unintended effects for other vehicles, such as cars and motorbikes.

The tunnel where the crash occurred opened in 1999, and the German automobile club ADAC gave it the second-best of six ranking levels in 2005.

"Such a severe and tragic accident must always be taken as an opportunity to analyze the factors that could have influenced the causes and effects of the disaster," said Mueller.

The bodies of the dead were repatriated with three Belgian military planes Friday.

Eight injured children traveled home with their relatives Thursday, and a regional hospital spokeswoman said 12 more were taken back by special medical transport flights Friday.

One child being treated at a hospital in the Swiss capital Bern will be flown home by the end of the weekend, spokeswoman Florence Renggli told reporters in Valais' main city of Sion.

A further three children are being treated at a specialist unit in nearby Lausanne.

"The priority for the hospital is to stabilize their vital functions," said Renggli.

The hospital in Lausanne said the three girls were put into an artificially induced coma because of multiple fractures and other serious injuries. Two of the girls with head injuries remained in a coma due to neurological problems, the hospital said in a statement, while the third with a spinal injury regained consciousness and was able to speak to others around her Friday.

"Our thoughts are for the friends of our daughters," the girls' unnamed parents were quoted as saying in the statement, referring to the other children who were on the bus. "We think a lot of them and their parents."

All EU flags hung at half staff along the EU Commission headquarters to mark the occasion.

At the KBC bank, automatic teller machines read: "We are speechless. All of KBC shares in the sorrow in silence."

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Broadcasters rescheduled their programming, with VTM postponing the live finale of its popular singing contest "The Voice of Flanders" for a day.

Special ceremonies were held at the two schools that shared the bus bringing pupils home from Switzerland. In Heverlee, the school gates were plastered with children's drawings about the tragedy. Primary school children stood in silence before releasing white balloons up in the sky.

On Thursday evening, hundreds packed the Holy Cross Church in Sierre, the city in southern Switzerland where the crash took place, for a memorial Mass.

John Heilprin and Frank Jordans in Geneva, Jeffrey Schaeffer in Sion, Switzerland and Mike Corder in The Hague, Netherlands, contributed to this story.