Three Adventure World workers, including British passport holder Andrew Lee, 30, were among those killed when a torrent of water engulfed the group of 53 at Interlaken.
District Attorney Michel-Andre Fels announced that the surviving five guides were being placed under formal investigation. They could face criminal charges. Locals claimed they ignored storm warnings.
The Swiss Tourist Board has announced a suspension of the sport until an investigation into the tragedy is complete.
Three British passport holders were among 21 thought to have perished on Tuesday. Nineteen bodies have been recovered. Rescuers were searching for two more.
The three UK passport holders were named as Mr Lee, Glyn Harries and Geoffrey Havard.
Mr Lee's girlfriend Robyn Kirby, who was from his home town of Rotura, New Zealand, but living in London, was on her way to Interlaken.
Mr Harries, 30, who emigrated to Perth, western Australia, had been visiting relatives in Chepstow before the trip.
As his parents Colin and Patricia and brother David, 32, prepared to travel to Switzerland, a family member, who did not wish to be named, said: "He was a wonderful son and his parents are devastated, everyone is.
"Glyn came over to England for the wedding of an old friend and stayed with us for about a week before leaving to go on this trip. He was really looking forward to it." Mr Havard's parents have also travelled to Switzerland.
Mr Havard, from Buckinghamshire, in his 30s, had been working for tour organisers Contiki for less than 12 months. He was driving one of the coaches carrying the group across Europe when he decided to take part in the expedition.
Relatives will be spared the trauma of identifying loved ones, many of whom had head injuries. But they have been asked to take medical records so that they can be used. Forensic expert Ulrich Zollinger said: "Their protective helmets and rubber shoes were all missing, which showed the incredible force of the flood."Reuse content