The driver of a Belgian bus that crashed in a Swiss alpine tunnel in March killing 22 children and six adults had heart problems but it was impossible to say if they caused the accident, investigators said in a report on Thursday.

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The 34-year-old driver was also taking a daily dose of an anti-depressant, the report of the post-mortem released Thursday by prosecutors in the canton of Valais said.

They said they had asked for a further investigation by pathologists to try to determine if either the heart condition or the medical treatment was a
factor in the crash which plunged Belgium into mourning.

The report said the cause of the unnamed driver's death was the traumatic injuries he received when the bus slammed into the tunnel wall at 100
kilometres (65 miles) an hour early on March 13th.

No traces of alcohol or narcotics were found in his blood and he had complied with requirements to take a break from driving.

While his left coronary artery was narrowed by at least 60 percent by sclerosis, "there is no evidence that this condition caused an attack which
would explain the loss of control of the vehicle," the report said.

Swiss prosecutor Olivier Elsig said in June that investigations were focusing on the driver, who had just taken over from a colleague when the
crash occurred.

"At this stage, the hypotheses of the involvement of a third vehicle, of a defect of the road or the tunnel, of the coach's excessive speed or of a
defect of the bus, have been excluded," he said.

"It would be ideal to pinpoint a reason. Perhaps we will never have one but we are getting much closer," he said.

The children who died were among 46 from two Belgian schools returning from a skiing holiday in the Alps. The adults were accompanying teachers and the two bus drivers.