By Jean-Francois Rosnoblet
SEYNE-LES-ALPES, France, March 25 (Reuters) - French investigators on Wednesday searched for the reason why a German Airbus ploughed into an Alpine mountainside, killing all 150 on board including 16 teenagers returning from a school trip to Spain.
Helicopters flew over the site where the A320 operated by Lufthansa's Germanwings budget airline disintegrated after it went down in a remote area of ravines en route to Duesseldorf from Barcelona. Police investigators made their way across the mountains on foot.
No distress call was received before the plane crashed on Tuesday, but French authorities said one of the two "black box" flight recorders, the cockpit voice recorder, has been recovered from the site 2,000 metres (6,000 feet) above sea level.
"The black box has been damaged. We will have to put it back together in the next few hours to be able to get to the bottom of this tragedy," French Interior Minister Bernard Cazeneuve told RTL radio, adding the box was still viable.
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.
French Civil aviation investigators are expected to hold a news conference on Wednesday afternoon.
In Washington, the White House said the crash did not appear to have been caused by a terrorist attack. Lufthansa said it was working on the assumption that the tragedy had been an accident, and any other theory would be speculation.
Germanwings was cancelling some flights on Wednesday as some crew members had refused to fly. "There are crew members who do not want to fly in the current situation, which we understand," a spokeswoman for Germanwings said.
"Seeing the site of the accident was harrowing," Lufthansa chief executive Carsten Spohr said on Twitter. "We will enable the relatives to grieve on site as soon as possible."
French President Francois Hollande will visit the area, about 100 km (65 miles) north of Nice, on Wednesday with German Chancellor Angela Merkel and Spanish Prime Minister Mariano Rajoy.
Germanwings believed 67 Germans were on the flight and Spain said 45 passengers had Spanish names. One Belgian was aboard, Australia said two of it nationals had died and Britain said it was likely some Britons were on the plane.
Also among the victims were 16 teenagers and two teachers from the Joseph-Koenig-Gymnasium high school in the town of Haltern am See in northwest Germany. They were on their way home after a week-long Spanish exchange programme near Barcelona.
The school held a day of mourning on Wednesday. Students arrived by bicycle and on foot like any normal day, but stopped by candles and flowers placed outside the school, where a hand-painted sign said: "Yesterday we were many, today we are alone."
Barcelona's Liceu opera house said two singers, Kazakhstan-born Oleg Bryjak and German Maria Radner, had died while returning to Duesseldorf after performing in Wagner's Siegfried at the theatre.
Aerial photographs showed smouldering wreckage and a piece of the fuselage with six windows strewn across the mountainside.
"We saw an aircraft that had literally been ripped apart, the bodies are in a state of destruction, there is not one intact piece of wing or fuselage," Brice Robin, prosecutor for the city of Marseille, told Reuters after flying over the site.
Germanwings said on Tuesday the plane started descending one minute after reaching cruising height and continued losing altitude for eight minutes.
Experts said that while the Airbus had descended rapidly, it did not seem to have simply fallen out of the sky.
A Lufthansa flight from Bilbao to Munich on Nov. 5 lost altitude after sensors iced over and the onboard computer, fearing the plane was about to stall, put the nose down. As a result, the European Aviation Safety Agency ordered a change in procedure for all A320 jets.
Asked whether something similar could have occurred on Tuesday, Germanwings Managing Director Thomas Winkelmann said, "At this time this evening, we are ruling out a possible cause in this area."
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.
It was the first disaster involving a large passenger jet in France since a Concorde crashed outside Paris nearly 15 years ago.
The A320 is one of the world's most used passenger jets and has a good safety record. According to data from the Aviation Safety Network, Tuesday's crash was the third most deadly involving the model. In 2007 a TAM Linhas Aereas A320 went off a runway in Brazil, killing 187 people, and 162 people died when an Indonesia AirAsia jet went down in the Java Sea in December.
The Germanwings plane was 24 years old, with engines made by CFM International, a joint venture between General Electric and France's Safran. (Additional reporting by Yann Le Guernigou, Maria Sheahan and Oliver Barth; Writing by David Stamp and Ingrid Melander; editing by Mark John and Giles Elgood)