A newspaper is reporting that the voice recorder indicates that one of the pilots was locked out of the cockpit before a Germanwings jetliner plummeted into a remote Alpine mountainside.

The New York Times is citing an investigator it doesn't identify as saying that the audio shows that after an ordinary start to the flight, one of the pilots left the cockpit and could not get back in.

The investigator told the newspaper that the pilot began knocking quietly on the door, then became more insistent, saying that eventually: "You can hear he is trying to smash the door down."

The investigator does not speculate as to why the other pilot didn't open the door or make contact with ground control before the plane crashed Tuesday. All 150 people aboard were killed.

A bus with 14 relatives of the Spanish victims who died in the French Alps plane crash has left Barcelona for an overnight journey that will take them to the crash area by Thursday.

Those going on the bus apparently did not want to take a Thursday morning flight from Barcelona to Marseille that is expected to shuttle many more relatives toward the site.

Spanish civil protection spokesman Sergio Delgado said all of the Spanish relatives will meet up in Marseille and head to the remote crash zone in Seyne-Les-Alpes together.

Spain's government has said at least 51 Spaniards were among the 150 victims of the crash. Airline Germanwings has said 35 of the 125 passengers identified were Spaniards.

The cockpit voice recorder had been badly damaged when the German jetliner smashed into the Alpine mountainside, and a two-minute span when the pilot lost contact, were vital clues to what caused the plane to go down, officials said on Wednesday.

Helicopters surveying the scattered debris lifted off at daybreak on Wednesday, 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.

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.