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.
A nation grieves for the 28 victims of the coach crash in Switzerland as a military aircraft has carried the bodies home.
First pictures have been released of the young victims of the Swiss bus crash. They are on display for people to pay their condolences.
A British boy was among the 28 people killed in a horrific bus crash in a Swiss tunnel.