French investigators have yet to find out why an Air France jet plunged into the Atlantic with the loss of 228 lives and the inquiry could take another year or year and a half, according to a top official.

The prospect of a lengthy inquiry will likely infuriate relatives of crash victims, some of whom have already begun to point the finger at the jet's air speed monitors, which they say were faulty and to blame for the deaths.

Paul-Louis Arslanian, the head of the BEA air accident investigation agency, said the hunt for the destroyed Airbus A330's black boxes would resume in autumn and that other countries would be invited to take part in the probe.

"For the moment, we can't explain the accident," he admitted at a breakfast for aviation journalists. "We still don't know what caused the AF447 accident."

A second BEA interim report will be ready within a "few weeks", he promised, but said this would not offer a definitive explanation, for which the families might have to wait "a year or a year-and-a-half".

Air France Flight 447 from Rio de Janeiro to Paris crashed into the ocean on June 1 and was ripped apart. Just before dropping off radar screens it had emitted a series of automatic warning signals indicating systems failures.

The wreckage of the jet and the bodies of some of the passengers and crew were recovered in deep water 1,000 kilometres off Brazil's coast, but the black box flight recorders remain lost.

Analysis of the error messages indicated a problem with the Airbus jet's "Pitot probes" - air speed monitors - but the BEA said in a July 2 interim report only that this was a "factor, not the cause" of the crash.

Since the crash, both the European air safety agency and planemaker Airbus have nevertheless advised airlines to replace the Pitot probes used on the doomed jet with a later and more reliable model made by a US firm.

Relatives of the dead have angrily demanded that Air France and Airbus take responsibility for the crash and French prosecutors have opened a preliminary manslaughter investigation that could lead to negligence charges.

In particular, they have alleged that Airbus and Air France knew about longstanding concerns over the A330's Pitots but had failed to replace them.