A recue helicopter flies over debris of the Germanwings passenger jet, scattered on the mountain side, near Seyne les Alpes, French Alps, March 24, 2015.
A recue helicopter flies over debris of the Germanwings passenger jet, scattered on the mountain side, near Seyne les Alpes, French Alps, March 24, 2015.

As the investigation into the crash Tuesday in France of a German airliner with 150 people on board continued, an investigator familiar with the contents of the cockpit voice recorder said one pilot apparently left the cockpit before the plane’s descent and was unable to get back in, the New York Times is reporting.

“The guy outside is knocking lightly on the door and there is no answer,” the investigator said. “And then he hits the door stronger and no answer. There is never an answer.”

He said, “You can hear he is trying to smash the door down.”

Officials investigating the crash of an Airbus A320 in the French Alps Tuesday said they have been able to extract a usable audio recording from a battered "black box" recovered at the site, France's aviation security investigator BEA said Wednesday.

“We just have been able to extract a usable audio data file,” Remi Jouty told a news conference earlier in the day, adding that it was too early to draw any conclusions about the causes of the crash. The black box was being examined in Paris, with authorities listening to the pilots' words in the final moments before the accident.

A "black box" voice recorder from the German Airbu
A "black box" voice recorder from the German Airbus operated by Lufthansa's Germanwings budget Airbus A320 crash is seen in this photo released by the BEA, France's air accident investigative agency, March 25, 2015.

“Detailed work will be carried on the file to understand interpret the voices and sounds that can be heard on the file,” he said, adding that he expected to have more analysis of the voices in “a matter of days.”

Earlier, French President Francois Hollande, German Chancellor Angela Merkel and Spanish Prime Minister Mariano Rajoy visited a makeshift rescue base near the Germanwings Airbus crash site Wednesday.

Hollande and Merkel flew over the crash site to see the devastation for themselves, then met with rescue workers. Rajoy also visited the center to be briefed on the grueling rescue operation where Flight 4U9525 crashed early Tuesday, scattering debris over a wide area. It is France’s worst aviation disaster in 15 years.

Hollande promised that French investigators would do everything to determine the cause of the crash that killed all 150 people aboard Germanwings Flight 4U 9525.

“Dear Angela, dear Mariano, rest assured ... we will find out everything and we will shed full light on the circumstances of this catastrophe,” Hollande told  Merkel and Rajoy. A majority of victims were Germans or Spaniards.

Hollande said the frame of a second black box had been found but that investigators were still searching for the box itself. However, Jouty would not confirm that any part of it had been recovered.

Cause of crash

Investigators said the remoteness of the crash site meant it could be days before a clear picture emerged of Tuesday's tragedy, the worst air disaster in France in 15 years.

Carsten Spohr, chief executive of Germanwing's parent company Lufthansa, told reporters on Wednesday that the crash appeared inexplicable, since the plane had no technical problems and the pilot was experienced.

A rescue helicopter flies over debris of the Germa
A rescue helicopter flies over debris of the Germanwings passenger jet, scattered on the mountain side, near Seyne les Alpes, French Alps, March 24, 2015.

Investigators will use the cockpit voice and flight data recorders to map out and focus their work, Alan Diehl, a former air safety investigator, told The Associated Press.

“Both will point you in directions of what is critical,” Diehl said. The four possible causes of any crash are human error, mechanical problems, weather, criminal activity or a combination of two or more.

Diehl said investigators will work backward, starting by eliminating what didn't happen.

Witnesses on the ground said they did not see smoke coming from the plane. One area resident, Sebastien Giroud, told French radio he watched as the aircraft lost altitude.

Giroud said he watched the plane for a few seconds, adding it was very low and that he knew right away there was a problem. Giroud said he was certain it would crash.  Later, he received word that it had.

Crash site 'was harrowing'

Lufthansa chief executive Carsten Spohr, who visited the crash site, tweeted: “Seeing the site of the accident was harrowing.

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

The plane went down Tuesday morning without issuing any distress signals, while en route from Barcelona, Spain, to the German city of Dusseldorf.

Map showing locations of Dusseldorf, Barcelona, an
Map showing locations of Dusseldorf, Barcelona, and Barcelonnette, with a satellite inset of the crash site

A definitive list of the 150 aboard the Germanwings Airbus plane that crashed into a rocky ravine in the Alps had yet to be produced, but it was already known the bulk of victims were German and Spanish, French Prime Minister Manuel Valls said Wednesday.

Among the passengers was believed to be a pair of German opera singers, Kazakhstan-born Oleg Bryjak and German Maria Radner, as well as 16 German teenagers returning from a school trip. Two Iranian sports journalists -- Milad Hojatoleslami, who worked for the Tasnim news site, and Hussein Javadi, a reporter for the Vatan Emrouz newspaper -- also were listed among the victims.

Booz Allen Hamilton, Inc., contractor Yvonne Selke and two other U.S. citizens were among the dead. State Department spokeswoman Jen Psaki confirmed that Selke and her daughter, Emily, were on the flight and said a third U.S. citizen whose name was being withheld "out of respect for the family," was on board.

Act of terrorism unlikely

Lufthansa, the parent company of the budget subsidiary Germanwings, said it is treating the crash as an accident at this time, and cautioned that any immediate references to possible terrorism are speculative.

Germanwing's A-320 Safety rating
Germanwing's A-320 Safety rating

French Interior Minister Bernard Cazeneuve said the fact debris was scattered over a small area of about one and a half hectares showed the plane likely did not explode in the air, meaning a terrorist attack was not the most likely scenario.

In Washington, the White House also said the crash did not appear to have been caused by a terrorist attack.?

Thomas Winkelmann, the airline's managing director, said radar showed the jetliner flying at 11,500 meters before entering a deadly 8-minute descent into the Alpes de Haute-Provence region, 100 kilometers north of Nice. He said radar contact was lost at 1,800 meters, and said the jetliner underwent routine inspection in Dusseldorf on Monday.

Germanwings: Victims include 72 Germans, 35 Spaniards

The aircraft came down in a region known for skiing, hiking and rafting, but which is difficult for rescue services to reach. The base of operations for the recovery effort was set up in a gymnasium in the village of Seyne-les-Alpes.

Winkelmann and others said land access to the snow-covered site is virtually impossible, and that rescue personnel reached the wreckage by helicopter.  Recovery efforts are expected to last at least a week.

Rescue helicopter pilot Xavier Roy, who is coordinating air operations, said, “When we go to a crash site we expect to find part of the fuselage. But here we see nothing at all."

Dozens of rescue workers combed the crash site --  steep mountainsides and a ravine -- as they looked for bodies.

To meet with families

Leaders of France, Germany and Spain also had planned to meet with family members in a makeshift chapel set up in a gymnasium, Hermitte said.

In Spain, flags flew at half-mast, meetings were canceled and workplaces fell silent Wednesday as people mourned the 150 crash victims, including at least 51 Spaniards. Spain declared three days of mourning and King Felipe VI cut short his first state visit to France on Tuesday just minutes after it began when he heard news of the tragedy.

U.S. President Barack Obama offered condolences to "our friends in Europe, especially the people of Germany and Spain." He also offered assistance from American officials.

Speaking in Parliament, British Prime Minister David Cameron also offered condolences on Tuesday's crash that killed 150.

"It is heartbreaking to hear about the schoolchildren, the babies, the families whose lives have been brought to an end," he said.

Earlier, German Foreign Minister Frank Walter-Steinmeier, who viewed the crash site by air, called the scene "a picture of horror."

Lacking distress calls, the French aviation authority said air traffic controllers declared an emergency because they lost contact with the cockpit.

Germanwings is a budget airline subsidiary of the German-based airline Lufthansa. The A320 aircraft that crashed was 24 years old and was at the upper end of its normal use as a commercial airliner. It had undergone a mechanical inspection on Monday.

Lisa Bryant contributed to this report from Paris. Some material came from Reuters, AFP and AP.