PARIS (AFP) - A technical fault, pilot error, or an act of terror - there are many theories over the mysterious crash of the Germanwings Airbus A320 in the French Alps on Tuesday that killed 150 people.
The black box has been found and is being delivered to investigators. They will also seek to glean information from the debris, victims' bodies, radio communications and maintenance logs, as well as looking at the background and training of the crew.
WHAT DOES THE LACK OF A DISTRESS CALL MEAN?
"The crew did not put out a Mayday call. It was air-traffic control that decided to declare the plane in distress after losing contact with the crew," said civil aviation authorities.
"It was the combination of the loss of radio contact and the descent that caused the control room staff to send out a distress call at 0930 GMT."
"The lack of a Mayday call opens up all possibilities," said a former investigator with France's Bureau of Investigation and Analysis for the Security of Civil Aviation (BEA), on condition of anonymity.
WHAT ARE THE MAIN THEORIES?
"For the moment, it could be a technical problem, a non-technical problem, a poor reaction by the crew to an emergency situation as was the case with the AF447 flight from Rio to Paris," said the aviation expert, referring to the Air France plane which crashed into the Atlantic in June 2009 after stalling in midair.
CAN WE RULE OUT A TERRORIST ATTACK?
"If a plane explodes in midair, the debris is spread across several kilometres as was the case with the Malaysia Airlines plane that was shot down over Ukraine."
That does not rule out the possibility that the plane was hijacked by people on board prior to the crash. That will only be ruled out by inspection of the wreckage and the black box recorder.
No other plane - whether civilian or military - has been listed as missing.
WAS THE AGE OF THE AIRCRAFT A FACTOR?
It is not the age of an aircraft that determines its vulnerability - it is the quality of the maintenance.
Leading aviation companies submit to stringent, frequent checks and pilots inspect the planes ahead of every flight.
This particular Airbus A320 of Germanwings underwent full maintenance in summer 2013, said the head of the company Thomas Winkelmann.
"But we cannot rule out a structural issue: a failure of a part of the structure caused by an absence of detailed maintenance or the wear of a particular element that will become apparent after tens of thousands of flight hours," said the former investigator.
"In the history of aviation, it's only when accidents occur that we are able to detect unforeseen weaknesses on parts of a plane where maintenance procedures were not thought necessary," the investigator added.