A rescue helicopter from the French Gendarmerie flies over the snow covered French Alps during a search and rescue operation near to the crash site of an Airbus A320, near Seyne-les-Alpes, March 25, 2015. [Photo/Agencies]

SEYNE-LES-ALPES, France - A cockpit voice recorder badly damaged when a German jetliner smashed into an Alpine mountainside and a crucial two-minute span when the pilot lost contact are vital clues into what caused the plane to go down, killing all 150 people on board, officials said 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 flight from Spain to Germany went into an unexplained eight-minute dive ahead of crashing Tuesday morning.

Crews were making their way slowly to the remote crash site through fresh snow and rain, threading their way to the craggy ravine. On Tuesday, the cockpit voice recorder was retrieved from the site, French Interior Minister Bernard Cazeneuve said.

"The black box is damaged and must be reconstituted in the coming hours in order to be useable," Cazeneuve told RTL radio.