More than three years after Air France Flight 447 plunged into the southern Atlantic Ocean, killing all 228 people aboard, authorities are preparing to release their final report on the fatal crash.

The aircraft's voice recorder and flight data recorder were found on the ocean floor in May 2011 after an extensive search using miniature submersible vehicles.

France's Bureau of Investigation and Analysis (BEA) said the data indicated that the flight from Rio de Janeiro to Paris crashed because the aircraft's speed sensors gave invalid readings.

When did Flight 447 go down?

Flight 447 -- which was en route to Paris from Rio de Janeiro -- made its last contact with Brazil's Atlantic Control Center (ACC) at around 01:33 GMT on June 1, 2009, informing the center of the plane's position as it crossed the Atlantic.

Soon after, Brazil's air control contacted Dakar's control center in North Africa and reported that AF 447 was entering an area on its route known for constant bands of severe turbulence, officials said.

There was no further contact with the plane.

Why has it taken so long to find the wreckage?

The area where the Airbus A330 went down is in the mid-Atlantic -- two to four days for ships to reach from the nearest ports in Brazil or Senegal in West Africa.

The U.S. National Oceanic and Atmospheric Administration estimates the ocean depth in the area at 3,000 meters (about 9,840 feet) to 7,500 meters (24,600 feet). Brazilian officials have said the sea depth in the area is around 2,000 to 3,000 meters (6,562 to 9,842 feet).

The latest search covered a 46-mile (75-kilometer) radius around the last known position of Flight 447, investigators said.

According to the BEA the underwater terrain in this area is rough with underwater mountains and valleys.

"It is a mountain range as big as the Alps," David Learmount of Flight International told CNN. "There was always the possibility that the wreckage from the aircraft disappeared down a crevasse. This is not a flat-bottom environment like the North Sea is."

"Therefore it is an unprecedented search in terms of depth and sub-sea terrain."

What would happen to wreckage at these depths?

Up until last year, the memory module of the aircraft's data recorder -- a vital device for crash investigators which records any instructions sent to the aircraft's electronic systems -- had been missing. As a result, some experts feared it would not work even if recovered.

"Pressure and corrosion have likely damaged the wreckage, especially as it has been down there for so long," said Learmount. "Flight recorders are not designed to withstand pressures of the depths that this aircraft actually went to."

But he warned that flight recorders recovered from other crash sites have not worked when they should have, with the reverse also true.

Do we know why Flight 447 crashed?

Last year's BEA report said the airplane climbed to 38,000 feet when "the stall warning was triggered and the airplane stalled." It then descended, crashing into the Atlantic. The descent lasted 3 minutes and 30 seconds and the engines remained operational, said the report.

Studies of the debris and bodies that were found soon after the crash led the BEA to conclude the plane hit the water belly first, essentially intact. Oxygen masks were not deployed, indicating that the cabin did not depressurize, the BEA said in a 2009 report.

Tests have already brought into question the performance of pitot tubes, which measure the pressure exerted on the plane as it flies through the air, and are part of a system used to determine air speed.

Before it crashed, Flight 447 sent out 24 automated error messages that suggested the plane may have been flying too fast or too slow through the thunderstorms, officials have said.

The European Aviation Safety Agency issued a directive in late August requiring airlines to replace pitot tubes manufactured by Thales Avionics on Airbus A330s and A340s. It said airlines should replace them with other Thales tubes and those manufactured by Goodrich.

The lack of speed, wind or direction information also prevented the Autopilot system from functioning, said air accident investigator Alain Bouillard said at the time of the crash. "This tells us that the plane has to be, in this case, directed by the pilot," he said.