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Air France flight 447 did not break up in the air but plunged vertically into the Atlantic Ocean, according to the French head investigator of last month's crash, which killed all 228 people on board.

Alain Bouillard said life vests found among the wreckage were not inflated, indicating the accident happened so quickly that the passengers had no time to react.

Speed sensors on the Airbus A330 flying from Rio de Janeiro to Paris were not to blame, he said, though "we are far from understanding the cause of the crash".

"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 have found "neither traces of fire nor traces of explosives."

One of the automatic messages emitted by the plane indicates it was receiving incorrect speed information from the external monitoring instruments, which could destabilise the control systems. Experts have suggested those external instruments might have iced over.

No information was being given out from autopsies of the bodies found, Bouillard told a news conference at the headquarters of the French air accident agency BEA in Le Bourget, outside Paris.

The chances of finding the flight recorders are falling as the signals they emit fade. Without them, the full causes of the accident may never be known. The automated messages sent by the plane before it fell gave rescuers only a vague location to begin their search. Bouillard said the search for the plane's black boxes has been extended by 10 days and would continue untill 10 July.

Families of the victims had been briefed before the media on the findings so far of the BEA investigation.

Earlier, Christophe Guillot-Noel, head of an association for the crash victims' families said they wanted all the facts, "above all to be able to avoid this eventually happening again".

"We have just one demand: transparency. We have just one expectation: the truth," he said.