Members of Brazil's military search for debris from an Air France jet over the Atlantic Ocean on Wednesday. (AP Photo/Brazil Defence Ministry)

Investigators continue to seek clues in the sudden crash of Air France flight 447 off the coast of Brazil, but certainty about the cause could remain elusive.

"We cannot rule out that we will not find the flight recorders," said Paul Louis Arslanian, the head of France's civil aviation agency, on Wednesday. "I cannot rule out the possibility that we might end up with a finding that is relatively unsatisfactory in terms of certainty."

The timeline of events has proved perplexing. Air France received an automated message from the plane at 10 p.m. ET on Sunday, indicating that an electrical malfunction had occurred. Air France said the plane had passed through "a thunderous zone with strong turbulence" shortly before the message was sent. The three experienced pilots on board did not send a mayday call before the plane disappeared. According to Arslanian, the plane did not have any problems prior to takeoff.

'Mystifying' process of recovery

Debris from Air France Flight 447 has been found about 650 kilometres northeast of the archipelago of Fernando de Noronha. (CBC)

Investigators hoped the recovery of debris would yield some information, though even this evidence has so far proved bewildering, said Wagdi Habashi, a professor of mechanical engineering at McGill University.

"It's still mystifying because we also hear that the debris separated in two places, that they're separated by 60 kilometres — so if that's true that would indicate that the airplane probably broke up in mid-flight, and that's why it's spread out over a large area," Habashi told CBC's Metro Morning on Wednesday.

"Some of the people are of the opinion that even if it broke up while hitting the water, it's still possible for the ocean currents to have separated the debris that far. It's still not clear."

Extreme weather

Air France officials said severe weather was likely a factor in the crash. At this time of the year, powerful Atlantic storms are common near the equator. While early speculation suggested lightning may have caused the plane to go down, aviation experts note that modern planes can typically handle such events.

"The aircraft today are built to withstand electrical discharge such as lightning," Mike Doiron, principal and CEO of the Moncton Flight College, told Metro Morning.

"Normally it's — I wouldn't want to call it a non-event because sometimes you do have some form of superficial damage or in some cases you might have some equipment damage — but invariably it's never anywhere near something that would cause a breakup in the flight like that."

Composite materials

Habashi agreed that planes are certified to stringent standards and usually can endure severe turbulence and extreme weather. But he noted that in rare circumstances lightning might strike an area of the plane that hasn't been tested extensively, and that a section of the plane built using composite materials might prove vulnerable.

This undated picture taken at Houston's George Bush Intercontinental Airport shows the Air France Airbus 330-200 that has been reported missing. (Airteamimages.com/Associated Press)

"In the good old days airplanes were all made of aluminum parts and aluminum is a very good conductor of electricity, so that lightning would hit in one place and would travel along the skin of the airplane and go out in another place," he said.

"Now with more and more plastic or reinforced plastic or composite that is used in airplanes, these things are not as good a conductor for electricity and you must add materials to it on the surface to be able to conduct electricity."

"It's quite possible that … some of this material might have been eroded by hail, by the storm and then lightning would have a totally different effect on that component, which is at the tail of the airplane."

Information about a possible electrical failure will not be known unless search crews can recover the plane's black box recorders. The black boxes — which have voice and data recorders — are feared to be in water thousands of metres deep at the bottom of the ocean floor.

Terrorism

Air France said on Wednesday that it received a bomb threat regarding a plane leaving Buenos Aires for Paris on May 27. Security officials checked the plane and no explosive devices were uncovered.

But France's Defence Minister Hervé Morin said there have been no indications the crash of Flight 447 was an act of terrorism.

Instrument malfunction

Investigators on Thursday said they were looking into the possibility that instruments used to collect data to measure airspeed and altitude may have malfunctioned. Officials told The Associated Press that an external probe used to gauge air pressure may have iced over. The aviation officials also theorized sensors inside the aircraft may have malfunctioned. This failure could have occurred as the plane entered turbulence, causing the pilots to lose control of the aircraft.

With files from the Associated Press