SEYNE-LES-ALPES, France (AP) — A black box recovered from the scene and pulverized pieces of debris strewn across Alpine mountainsides held clues to what caused a German jetliner to take an unexplained eight-minute dive Tuesday midway through a flight from Spain to Germany, apparently killing all 150 people on board.

The victims included two babies, two opera singers and 16 German high school students and their teachers returning from an exchange trip to Spain. It was the deadliest crash in France in decades.

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 descent. The pilots sent out no distress call and had lost radio contact with their control center, France's aviation authority said, deepening the mystery.

While investigators searched through debris from Flight 9525 on steep and desolate slopes, families across Europe reeled with shock and grief. Sobbing relatives at both airports were led away by airport workers and crisis counselors.

"The site is a picture of horror. The grief of the families and friends is immeasurable," German Foreign Minister Frank-Walter Steinmeier said after being flown over the crash scene. "We must now stand together. We are united in our great grief."

It took investigators hours to reach the site, led by mountain guides to the craggy ravine in the southern French Alps, not far from the Italian border and the French Riviera.

Video shot from a helicopter and aired by BFM TV showed rescuers walking in the crevices of a rocky mountainside scattered with plane parts. Photos of the crash site showed white flecks of debris across a mountain and larger airplane body sections with windows. A helicopter crew that landed briefly in the area saw no signs of life, French officials said.

"Everything is pulverized. The largest pieces of debris are the size of a small car. No one can access the site from the ground," Gilbert Sauvan, president of the general council, Alpes-de-Haute-Provence, told The Associated Press.

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"This is pretty much the worst thing you can imagine," said Bodo Klimpel, mayor of the German town of Haltern, rent with sorrow after losing 16 tenth graders and their two teachers.

The White House and the airline chief said there was no sign that terrorism was involved, and German Chancellor Angela Merkel urged reporters not to speculate on the cause.

"We still don't know much beyond the bare information on the flight, and there should be no speculation on the cause of the crash," she said in Berlin. "All that will be investigated thoroughly."

Lufthansa Vice President Heike Birlenbach told reporters in Barcelona that for now "we say it is an accident."

In Washington, the White House said American officials were in contact with their French, Spanish and German counterparts. "There is no indication of a nexus to terrorism at this time," said U.S. National Security Council spokeswoman Bernadette Meehan.

Merkel, French President Francois Hollande and Spanish Prime Minister Mariano Rajoy were to visit the site Wednesday.

French Interior Minister Bernard Cazeneuve said a black box had been located at the crash site and "will be immediately investigated." He did not say whether it was the flight data recorder or the cockpit voice recorder.

The two devices — actually orange boxes designed to survive extreme heat and pressure — should provide investigators with a second-by-second timeline of the plane's flight.

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 captures 25 hours' worth of information on the position and condition of almost every major part in a plane.

Germanwings is low-cost carrier owned by Lufthansa, Germany's biggest airline, and serves mostly European destinations. Tuesday's crash was its first involving passenger deaths since it began operating in 2002. The Germanwings logo, normally maroon and yellow, was blacked out on its Twitter feed.

Lufthansa CEO Carsten Spohr called it the "blackest day of our company's 60-year history." He insisted, however, that flying "remains after this terrible day the safest mode of transport."

Germanwings said 144 passengers and six crew members were on board. Authorities said 67 Germans were believed among the victims, including the 16 high school students and two opera singers, as well as many Spaniards, two Australians and one person each from the Netherlands, Turkey and Denmark. In Japan, the government said two Japanese citizens were believed to be on the plane.

Contralto Maria Radner was returning to Germany with her husband and baby after performing in Wagner's "Siegfried," according to Barcelona's Gran Teatre del Liceu. Bass baritone Oleg Bryjak had appeared in the same opera, according to the opera house in Duesseldorf.

The plane left Barcelona Airport at 10:01 a.m. and had reached its cruising height of 38,000 feet when it suddenly went into an eight-minute descent to just over 6,000 feet, Germanwings CEO Thomas Winkelmann told reporters in Cologne.

"We cannot say at the moment why our colleague went into the descent, and so quickly, and without previously consulting air traffic control," said Germanwings' director of flight operations, Stefan-Kenan Scheib.

At 10:30, the plane lost radio contact with the control center but "never declared a distress alert," Eric Heraud of the French Civil Aviation Authority told the AP.

The plane crashed at an altitude of about 6,550 feet (2,000 meters) at Meolans-Revels, near the popular ski resort of Pra Loup. The site is 430 miles (700 kilometers) south-southeast of Paris.

"It was a deafening noise. I thought it was an avalanche, although it sounded slightly different. It was short noise and lasted just a few seconds," Sandrine Boisse, the president of the Pra Loup tourism office, told the AP.

Authorities faced a long and difficult search-and-recovery operation because of the area's remoteness. The weather, which had been clear earlier in the day, deteriorated Tuesday afternoon, with a chilly rain falling. Snow coated nearby mountaintops.

French Interior Ministry spokesman Pierre-Henry Brandet said the crash site covered several acres, with thousands of pieces of debris, "which leads us to think the impact must have been extremely violent at very high speed."

Search operations were suspended overnight and were to resume at daybreak, though about 10 gendarmes remained in the desolate ravine to guard the crash site, authorities said.

Winkelmann said the pilot, whom he did not name, had more than 10 years' experience working for Germanwings and its parent airline Lufthansa.

Florian Graenzdoerffer Lufthansa Spokesman for North Rhine Westphalia said the company had to cancel seven flights out of Dusseldorf because a number of crew members felt they were unfit to fly following news of the accident.

"I can't tell you any details because this is a personal decision and in our business we have an agreement if a crew feels unfit to fly ... then we respect this," Graenzdoerffer said.

The aircraft was delivered to Lufthansa in 1991, had approximately 58,300 flight hours in some 46,700 flights, Airbus said. The plane underwent a routine check in Duesseldorf on Monday, and its last regular full check took place in the summer of 2013.

The A320 plane is a workhorse of modern aviation, with a good safety record.

The last time a passenger jet crashed in France was the 2000 Concorde accident, which left 113 dead.

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Charlton reported from Paris. Associated Press writers Lori Hinnant, Thomas Adamson and Elaine Ganley in Paris; Claude Paris in Seyne-les-Alpes; David McHugh in Frankfurt; Geir Moulson and David Rising in Berlin; Frank Augstein in Duesseldorf; Al Clendenning in Madrid; Joe Wilson in Barcelona; Kirsten Grieshaber in Haltern, Germany, and AP Airlines writer Scott Mayerowitz in New York contributed to this report.