Prosecutor Olivier Elsig cast doubt on reports that the driver had been busy loading a DVD player just before the crash.
He said: "A dozen interviews with surviving children have been carried out, without any information provided pointing to a cause or causes of the accident.
"Regarding the theory of inattention due to a DVD being inserted, none of the those heard saw the driver doing anything like this."
Investigators have established that no other vehicle was involved, that the bus was not speeding, that the driver had not drunk alcohol, and he had not had a heart attack.
Sky News reports that Switzerland's prosecutor-general, Olivier Elsig, has said that neither the speed of coach or excess alcohol in driver were the cause of the fatal accident in the Alps.
Pupils and parents have released dozens of white balloons from St Lambertus school in Heverlee where some of the children came from.
Belgian military aircraft have flown home the bodies of 22 children and six adults killed in the Swiss bus crash. Hercules transport aircraft carrying white coffins arrived in Melsbroek airport.
Belgium has fallen silent to remember the 28 people killed in Tuesday's crash.