How could it have happened?

Investigators in Austria have revealed that the doomed ski train suffered technical problems before it entered the mountainside tunnel killing 159 people.

Debris found on the rails before the tunnel entrance had apparently come from the vehicle, and indicated that a fire could have broken out before it went into the tunnel.

Christian Tisch, a police forensic technician said "elements recovered from the site had been removed for chemical analysis", giving the first clues about events preceding the tragedy.

The funicular underwent safety checks only two months ago

The investigators' findings confirmed accounts given by some of the 12 survivors of the tragedy who said they saw smoke before the train entered the tunnel. The driver reportedly noticed the blaze, but it was too late to stop the train going in.

But Mr Tisch said the train inexplicably stopped about 600 metres into the 3.2km-long tunnel to the Kitzsteinhorn glacier.

The driver is said to have immediately contacted the base who had been unable to do anything to restart the funicular.

The incident has left experts puzzled at how an engineless train pulled by a cable could have caught fire in the first place.

Emergency exists

Many theories for the fire have been put forward, inlcuding:

Diesel fuel was being carried on the train.

Firecrackers were on board.

Cigarette or cooking gas canisters were to blame.

Some believe that flammable ski equipment and clothes may have fuelled the blaze.

Smoke was sucked through the tunnel like a giant chimney

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 air in from the bottom and sending toxic smoke billowing upwards.

Passengers were trapped between the fire below them and the smoke-filled tunnel with no clearly-marked emergency exits.

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.

'Doors had jammed'

There is also confusion over why the doors apparently sealed the passengers into the burning train.

They screamed as they tried to prise open the doors and smash the windows

German survivor

Manfred Mueller, the head of technical operations for the underground cable car system 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 said the doors had jammed, trapping passengers inside.

There are questions as to why the train's doors jammed

"They screamed as they tried to prise open the doors and smash the windows," he said.

"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.