Some believe that flammable ski equipment and clothes may have fuelled the blaze.
Experts say they are puzzled at how an engineless train pulled by a cable could have caught fire in the first place.
"We have received information that the light of a fire was already visible to outside witnesses as the train was entering the tunnel," said Erik Buxbaum, Austria's public security chief.
He said the driver had noticed the blaze, but it was too late.
Many rumours have circulated locally: that diesel fuel was being carried on the train; that firecrackers were on board; that a cigarette or cooking gas canisters were to blame; even that a bomb had been planted.
Some believe that flammable ski equipment and clothes may have fuelled the blaze.
Whatever the cause, officials admit the blaze caught them unawares.
"It wasn't provided for... we were taken by surprise," said the head of technical operations for the underground cable car system, Manfred Mueller.
What is clear is that, once the blaze took hold, passengers stood no chance of escape.
The steep tunnel acted like a giant chimney, sucking oxygen in from the bottom and sending toxic smoke billowing upwards.
Some passengers who got out of the front of the train are believed to have been choked by the fumes as they clambered upwards through the dark, smoke-filled tunnel.
The few who fled downwards from the rear of the train were the only ones to survive.
There is also confusion over why the doors apparently sealed the passengers into the burning train.
Mr Mueller said the train driver was told to open all doors after an alarm sounded, and it was another five or 10 minutes before radio contact was suddenly lost.
But one German survivor told Austria's APA news agency the doors had jammed, trapping passengers inside.
"All I wanted was to get out and I only managed to escape by the skin of my teeth because a window was kicked open, letting me battle my way out."
The tunnel, which opened in 1974, is one of the earliest constructions of its kind in the world, but the train itself was modern.
Inspectors had carried out safety checks as recently as September.
"Everything was fine," said regional governor Franz Schausberger.
The tragedy focuses attention again on the safety of Alpine tunnels.
Last year, dozens of people died in fires in the Mont Blanc tunnel linking France and Italy, and in the Tauern tunnel not far from the scene of Saturday's tragedy.