The Air France Airbus that crashed into the Atlantic killing 228 people plunged vertically into the ocean intact and at high speed, investigators said yesterday.

Alain Bouillard, leading the investigation into the crash on June 1 said the plane's speed sensors, called Pitot tubes, were not the direct cause of the crash but a factor in it.

“It is an element but not the cause,” Mr Bouillard said announcing initial findings of the crash inquiry in Paris. “Today we are very far from establishing the causes of the accident.”

The Airbus A330-200 flying from Rio de Janeiro to Paris went down in a remote area of the Atlantic, 930 miles off Brazil's mainland and far from radar coverage.

Dr Eithne Walls, from Ballygowan in Co Down, died in the crash alongside two of her friends from the Republic.

The French investigation agency BEA called it one of history's most challenging plane crash investigations.

One of the automatic messages sent by the plane indicates it was receiving incorrect speed information from the external monitoring instruments, which could destabilise its control systems. Those external instruments may have iced over.

Mr Bouillard said the plane “was not destroyed in flight”.

“The plane seems to have hit the surface of the water on its flight trajectory with a strong vertical acceleration,” he said, adding that investigators found “neither traces of fire nor traces of explosives”.

Mr Bouillard said life vests found among the wreckage were not inflated, suggesting the passengers were not prepared for a crash landing in the water. The pilots apparently also did not send any mayday calls.

A burst of automated messages emitted by the plane before it fell gave rescuers only a vague location to begin their search, which has failed to locate the plane's black boxes in the vast ocean.

The chances of finding the flight recorders are falling daily as the signals they emit fade.

Without them, the full causes of the tragic accident may never be known.

He said the search for the plane's black boxes has been extended by 10 days and will continue until July 10.

The black boxes — in reality bright orange — are resting on an underwater mountain range filled with crevasses.