French investigators will release on Thursday a final report into what caused the mid-Atlantic crash of a Rio to Paris Air France flight that killed 228 people in 2009.

It may blame a combination of human error and technical malfunction. This was the finding of a preliminary report and also, according to a source, of a separate judicial enquiry to be released next week.

The report to be published Thursday by the French aviation safety authority BEA is eagerly awaited amid a bitter row between Airbus and Air France over who ultimately bears responsibility

The Airbus A330 vanished at night and during a storm on June 1, 2009. It took days before debris was located in the remote equatorial Atlantic Ocean area, and far longer until the wreckage was recovered.

The black boxes were finally located by robot submarines after a search spanning 23 months and costing about 32 million euros ($40 million).    

The BEA in a preliminary report into the crash last July said the pilots had failed to react correctly when the Airbus jet stalled and lost altitude after its speed sensors froze up and failed.

The aircraft had entered turbulence two hours into the flight when the autopilot suddenly disengaged, it said.

Shortly after, an instrument showed a sharp fall in airspeed and a stall warning sounded, indicating the plane was no longer being lifted by the air pushing under the wings.

As the two co-pilots struggled to understand the situation, the captain, who had left the cockpit to take a rest, returned but did not retake control of the plane, the BEA said last year.

Air France has insisted the pilots were not to blame, saying the stall alarm had malfunctioned.

French magistrates are investigating Air France and Airbus for alleged manslaughter in connection with the crash, notably because of the malfunctioning speed sensors, known as Pitots.

The airline replaced the Pitots, manufactured by French company Thales, on its Airbus planes with a newer model after the crash.

Victims' families have previously alleged that the involvement of big French corporations such as Airbus and Air France was influencing the probe into the causes of the disaster. 

A separate judicial report – due to be presented to victims' families next Tuesday – has concluded that both pilot error and malfunctioning speed sensors were responsible, a source has told AFP.

The source said the 356-page judicial report had found that speed sensors froze up and failed, but also that the "captain had failed in his duties" and "prevented the co-pilot from reacting".