A cockpit voice recorder badly damaged when a German jetliner smashed into an Alpine mountainside, and a two-minute span when the pilot lost contact, are vital clues to what caused the plane to go down, killing all 150 people on board, officials said on Wednesday.

Helicopters surveying the scattered debris lifted off at daybreak, hours ahead of the expected arrival of bereaved families and the French, German and Spanish leaders.

The Germanwings Airbus A320 was on a flight from Barcelona to Dusseldorf when it went into an unexplained eight-minute dive before crashing on Tuesday morning.