( CNN ) -- Twenty-two children and six adults died when a bus crashed into the wall of a tunnel in Switzerland , police said . Another 24 children were injured in Tuesday night 's wreck , authorities said . Three children are hospitalized in critical condition , Jean-Pierre Deslarzes , medical chief of the Canton Valais rescue organization , told a news conference Wednesday afternoon . Others are expected to be discharged from the hospital in the coming days . The bus was on its way back to Belgium after a ski trip when it slammed into the side of a highway tunnel in Sierre in the Swiss canton of Valais . It was carrying 52 people : two drivers , four other adults and 46 children , aged 11 and 12 years old . Prosecutor Olivier Elsig told reporters at the news conference that the cause of the crash was still under investigation , but initial inquiries indicated the bus , which was new , was not speeding and that the road conditions were good . Police have said the speed limit in the tunnel was 100 kilometers per hour ( 62 mph ) . Experts are studying CCTV images and speaking to witnesses and some of those on the bus who were less badly injured to find out what happened , Elsig said , adding that no other vehicle was involved in the crash . An autopsy will be carried out on the bus driver , who is among the dead . Investigators are focusing on three potential causes for the accident , he said -- a technical issue with the bus , a health problem with the driver or human error . Rescuers reported that the children were wearing their seat belts , he said , but the impact of the crash was so great that the belts did not save their lives . Christian Varone , police chief for Valais Canton , said investigators were still working to confirm the identities of the 28 people killed . All but two of those injured had been identified and their families contacted , he said . `` This is a tragic day for Belgium , '' Belgian Prime Minister Elio Di Rupo said . He said there were 10 people of Dutch nationality among the victims as well as one of German and one of Polish origin . The survivors included 12-year-old twins , a girl and a boy , whose parents are a Spanish Catalan father and a Belgian mother , the Barcelona newspaper La Vanguardia reported Wednesday , citing family sources . The twins , identified as Alba and Marc , are out of danger and were treated in a hospital about 30 kilometers from the Swiss town of Sion , the newspaper reported . The parents were able to talk to them Wednesday and confirmed that they are safe and healthy . The family has gone to a hotel in Sion , the newspaper reported . The family moved from Spain to Belgium two years ago , the paper said . No problems had been reported with the bus , and it had passed technical checks in October , Di Rupo said . The drivers had respected regulations regarding rest time , he added . Pierre-Martin Moulin , assistant to the chief of police in Valais , earlier said authorities would need time to understand what had led the bus to hit the curb on the right side of the tunnel , then crash headlong into the wall of an emergency stopping point . It was too early to speculate about the cause of the accident , he told CNN from Sion , Switzerland . The tunnel was built recently and had many safety features , he added . The bus was returning from Anniviers in Switzerland when it crashed , said an online statement from the police in Valais . The children on the bus attended two schools in the Belgian towns of Heverlee and Lommel , it said . Deslarzes said the rescue operation had been `` very painful '' for the emergency workers because of the number of children involved . `` It was a difficult night for all the people engaged on the scene , '' Moulin said , expressing his condolences to families . He said the rescuers , who numbered more than 200 , had gotten quickly to the crash site but nothing could be done for the many children who were killed . `` To end a ski trip like that . ... There 's no words to describe what happened there , '' he said . Belgium has made available two military planes to take parents to see their injured children and to bring them back , Vandeweyer said . The Swiss Assembly held a minute 's silence Wednesday morning for the victims . The European Parliament also fell silent to mark the tragedy . Martin Schulz , the assembly 's president , paid tribute to the rescue workers and expressed his condolences to the families . The local government in Valais also paid tribute to those killed and their families . `` That a holiday trip can turn into such a tragedy is met by the people of the Canton Valais with deep sympathy and mourning , '' a statement on its website said . CNN 's Laura Smith-Spark , Stefan Simons , Samira Said and Al Goodman contributed to this report .
NEW : The bus was not speeding at the time of the crash , a Swiss prosecutor says . NEW : The impact was so great that their seat belts did not save the children , he says . Belgian Prime Minister : " I am shocked by this accident and deeply saddened to learn about it. The bus was returning to Belgium from a ski trip with 52 people aboard , most of them children .