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.

The cockpit voice recorder was retrieved from the site on Tuesday, French Interior Minister Bernard Cazeneuve said.

"The black box is damaged and must be reconstructed in the coming hours in order to be usable," Cazeneuve said.

Segolene Royal, a top government minister whose portfolio includes transportation, said that what happened at 10:30 am and 10:31 am is key to the investigation. After this, controllers were unable to make contact with the plane.

The flight data recorder, which Cazeneuve said had not yet been retrieved, captures 25 hours' worth of information on the position and condition of almost every major part of an aircraft.

Royal and Cazeneuve both stressed that terrorism is considered unlikely. Lufthansa said on Wednesday it could not explain why its Germanwings Airbus had crashed.

"It is inexplicable this could happen to a plane free of technical problems and with an experienced, Lufthansa-trained pilot," Lufthansa chief executive Carsten Spohr told reporters in Frankfurt.

Lufthansa said the 24-year-old plane had repairs on Monday to the hatch that houses the nose wheel. A spokeswoman said that was not a safety issue but that repairs had been done to reduce noise.

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

Students at the main high school in the German town of Haltern were gathering at an ever-increasing memorial of candles and flowers to mourn the loss of the 16 classmates and two teachers.

Fourteen-year-old Lara Beer said her best friend, whom she named only as Paula, was aboard the aircraft. She said she was waiting for the train her friend was supposed to be on, but went home when she did not turn up. "That's when my parents told me Paula was dead."

AP - Reuters

(China Daily 03/26/2015 page1)