The pilots sent out no distress call and had lost radio contact with their control centre, France's aviation authority said, deepening the mystery over the A320's mid-flight crash after a surprise 8-minute descent.
"The site is a picture of horror. The grief of the families and friends is immeasurable. We must now stand together. We are united in our great grief," German Foreign Minister Frank-Walter Steinmeier said in a statement after being flown over the crash scene and briefed by French authorities.
The crash left officials and families across Europe reeling in shock. Sobbing, grieving relatives at both airports were led away by airport workers and crisis counsellors. One German town was rent with sorrow after losing 16 high school students coming back from an exchange program in Spain.
"This is pretty much the worst thing you can imagine," a visibly rattled Haltern Mayor Bodo Klimpel said at a hastily called press conference.
After night fell on the hard-to-reach site, French authorities called off the search and helicopters stopped flying over the area.
About 10 gendarmes will spend the night at the crash site to guard it, and search operations will resume at daybreak, Lt. Col. Jean-Marc Meninchini of the regional police rescue service, told The Associated Press in the mountain town of Seyne-les-Alpes. Recovery operations are expected to last a week, he said.
German Chancellor Angela Merkel insisted, "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.
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.
Video and photos of the site showed scattered white flecks across a stony mountain and several larger airplane body sections with windows. French officials said a helicopter crew that landed briefly in the area saw no signs of life.
"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.
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 a data recorder or a cockpit voice recorder.
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.
Germanwings said Flight 9525 carried 144 passengers, including two babies, and six crew members. Officials believe 67 Germans were on board, including the 16 high school students from Haltern and two opera singers.
Barcelona's Gran Teatre del Liceu says German contralto Maria Radner was aboard the crashed plane along with her husband and baby. The opera house in Duesseldorf said bass baritone Oleg Bryjak, was also on the plane.
One Dutch citizen and one Danish citizen were among the victims as well, their governments said.
The plane left Barcelona Airport at 10:01 a.m., then began descending again shortly after reaching its cruising height of 38,000 feet, Germanwings CEO Thomas Winkelmann told reporters in Cologne. The descent lasted eight minutes.
Eric Heraud of the French Civil Aviation Authority said the Germanwings plane lost radio contact with a control centre at 10:30 a.m. Tuesday, but "never declared a distress alert itself." He said the combination of loss of radio contract and the plane's quick descent prompted the control centre to declare a distress situation.
"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.
The plane crashed at an altitude of about 2,000 metres (6,550 feet) near the towns of Prads-Haute-Bleone and Meolans-Revels and the popular ski resort of Pra Loup. The site is 700 kilometres (430 miles) 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.
Interior Ministry spokesman Pierre-Henry Brandet told BFM television he expected "an extremely long and extremely difficult" search-and-rescue operation because of the area's remoteness. The weather in the area deteriorated Tuesday afternoon, with a chilly rain falling.
Winkelmann said the pilot, whom he did not name, had more than 10 years' experience working for Germanwings and its parent airline Lufthansa.
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 plane had a minor technical problem Monday with a nose gear landing door, Lufthansa spokesman Christoph Meier said, but added that it was really only a noise problem that did not appear to have any link to the crash. He also said a few Germanwings crews asked not to fly after the crash "for personal reasons."
The safest part of a flight is normally when the plane is at cruising elevation. Just 10 per cent of fatal accidents occur at that point, according to a safety analysis by Boeing. In contrast, takeoff and the initial climb accounts for 14 per cent of crashes and final approach and landing accounts for 47 per cent.
In Paris, French President Francois Hollande called the crash "a tragedy on our soil."
The last time a passenger jet crashed in France was the 2000 Concorde accident, which left 113 dead — 109 in the plane and four on the ground.
Merkel spoke with both Hollande and Spanish Prime Minister Mariano Rajoy about the crash, immediately cancelling all other appointments.
"The crash ... is a shock that plunges us in Germany, the French and the Spanish into deep sorrow," said Merkel, who planned to travel to the region Wednesday.
The A320 plane is a workhorse of modern aviation. The single-aisle, twin-engine jet is used to connect cities between one and five hours apart. It is certified to fly up to 39,000 feet but it can begin to experience problems as low as 37,000 feet.
The A320 family also has a good safety record, with just 0.14 fatal accidents per million takeoffs, according to a Boeing safety analysis.
Hinnant contributed from Paris. Thomas Adamson and Elaine Ganley in Paris, 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 from Haltern, Germany, and AP Airlines writer Scott Mayerowitz in New York contributed.