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.

Key to the investigation is what happened in the minute after 10:30, said Segolene Royal, France's energy minister. From then, controllers were unable to make contact with the plane.

The voice recorder takes audio feeds from four microphones within the cockpit and records all the conversations between the pilots, air traffic controllers as well as any noises heard in the cockpit. The flight data recorder, which Cazeneuve said had not been retrieved yet, captures 25 hours' worth of information on the position and condition of almost every major part in a plane.

Royal and Cazeneuve both emphasized that terrorism is considered unlikely.

Victims included two babies, two opera singers, an Australian mother and her adult son vacationing together, and 16 German high school students and their teachers returning from an exchange trip to Spain.

The Airbus A320 operated by Germanwings, a budget subsidiary of Lufthansa, was less than an hour from landing in Duesseldorf on a flight from Barcelona when it unexpectedly went into a rapid 8-minute descent on Tuesday. The pilots sent out no distress call and had lost radio contact with their control center, France's aviation authority said.

Germanwings said 144 passengers and six crew members were on board.