Something happened on board Germanwings Flight 4U9525 that caused a loss of control of the plane, aviation journalist Anthony Davis told ITV News.
"The issue is why an aeroplane that is one of the safest type in the world was cruising quite happily at 38,000 feet, and was then put into a rapid descent," he said.
During a visit to the crash site, the boss of Lufthansa has avoided reporters' questions about the co-pilot's known depression.
Newly-released footage shows Germanwings co-pilot Andreas Lubitz in flight around ten years ago during his time as a trainee.
Pilots with a history of depression should not be banned from flying, a leading British psychiatrist has said.