"As things stand, there is considerable evidence that it was a result of outside intervention," State Criminal Police Office (LKA) investigator Mark Hammerl told the Austrian Press Agency. "Whether it was a case of negligence or arson is unclear."
"We are investigating all possibilities. Currently it is certain only that there was no technical reason," said Hammerl.
Investigators found no trace of fire accelerant, but materials present in the attic were highly combustible, including 20 propane tanks and insulation materials for planned refurbishment work.
So far there are no indications of a possible perpetrator. The investigation will continue over the coming days, and local residents will be questioned.
The fire broke out on Sunday evening at 8pm and within a short time the roof was consumed by flames.
Three gas bottles exploded, sending debris flying to a radius of 80 metres.
Fire fighters brought the flames under control by around 3am on Monday morning but continued damping down until Monday afternoon.
Three hundred people, including residents of nearby houses were temporarily evacuated.
Around 20 people spent the night in the gymnasium of the new middle school. The remainder stayed with relatives and friends.
According to police, 18 residents had to be rescued, nine of whom were admitted to Innsbruck hospital suffering from smoke inhalation.