SEYNE, France—An Airbus A320 carrying 150 people from Barcelona to Düsseldorf crashed into an alpine mountainside Tuesday after an unexplained 8-minute descent, leaving no apparent survivors and two countries in mourning.
Officials who flew over the crash site in the French Alps on Tuesday described an awful scene of desolation: a mountain face blanketed by small debris and body parts, suggesting the plane operated by Germanwings, the low-cost arm of Deutsche Lufthansa AG, disintegrated on impact.
“The violence of the crash leaves little hope for survivors,” French Interior Minister Bernard Cazeneuve said upon returning to Seyne, the remote village where emergency workers have established a base camp, after inspecting the site by helicopter as France rushed to mount a complex search-and-recovery operation.
The crash of Flight 9525 stunned Germany and Spain, whose nationals made up most of the flight manifest. German Chancellor Angela Merkel said she would join Spanish Prime Minister Mariano Rajoy in Seyne on Wednesday to pay respects to the victims alongside French President François Hollande.
The crash exacted a heavy toll on a small western German town, Haltern am See, which lost 16 teenagers and two teachers. The group was returning from a student exchange at a public high school near Barcelona.
“This is the darkest day in the history of our town,” said Bodo Klimpel, the town’s mayor.
Opera singers Oleg Bryjak and Maria Radner were also among the victims on board the plane, having performed in Barcelona.
Germanwings said the plane reached an altitude of 38,000 feet at 10:45 a.m., around the same time as it reached French airspace, then began an unplanned descent that ended in the crash.
French air-traffic controllers made multiple attempts to contact the flight but got no response, according to Roger Rousseau, the secretary-general for the SNCTA union of air-traffic controllers. “There was no distress call from the plane. No mayday call, no squawk from the transponder—nothing,” he said.
When the plane dipped out of radar’s view, the air-traffic controllers issued an alarm signal, which alerted military and police authorities. Moments later, the French military ordered a fighter jet to race to the area, according to a person familiar with the matter.
Mr. Rousseau said the plane didn’t deviate from its course as it lost altitude, which is an unusual pattern for an aircraft in distress.
“If there’s a loss of control, pilots usually lose their way too,” the union delegate said. “That didn’t happen in this case.”
Germanwings, a low-fare brand created by Lufthansa in 2002, had an unblemished safety record until Tuesday.
France mobilized more than 600 police and military personnel, together with 10 helicopters and a military plane, for the recovery effort. But with no direct road access to the crash site, emergency workers faced difficult options: hourslong treks at altitudes of 6,500 feet or rappelling from helicopters unable to land on treacherous mountainous terrain.
“It is really hard to get there,” said Damien Bon, a marshal with the local mountain police.
Nevertheless, French authorities said that they had recovered one of the two black box recorders from the Germanwings aircraft, and the BEA, France’s flight safety agency, was set to begin analyzing the contents. The recording devices typically provide the best clue why a plane crashed; officials didn’t say whether it was the flight data recorder or the cockpit voice recorder that was recovered.
Meantime, a dozen coroners were dispatched to Seyne to prepare for collecting body samples and helping to identify victims.
As darkness fell, several mountain rangers remained stationed in the area to secure the crash scene, as authorities suspended the search effort for the night.
Flight 9525’s eight-minute descent, combined with the lack of pilot communication, differentiates its crash from the most common types of accidents over the years in which modern jetliners have smashed into mountains or water, deepening the initial mystery for investigators. What is unclear is whether pilot input, a technical issue or something else triggered the descent that continued for 32,000 feet until the plane’s impact.
The highly automated A320’s flight-control computers, like those on other Airbus jets, have suffered in-flight incidents in which the plane suddenly pitches down. In November, for instance, a Lufthansa Airbus A321 dropped 3,000 feet in less than a minute before the pilots recovered, and European safety officials issued pilots instructions on how to react to avoid losing control of similar aircraft, including the A320. Germanwings on Tuesday said the plane that crashed complied with all the latest safety requirements.
Sudden depressurization or structural failure—anything piercing the skin of the airplane fuselage—can cause a rapid descent and possibly knock out pilots so that they can’t communicate. But based on past incidents, such a descent would be expected to be steeper than that of Flight 9525, which lost altitude at roughly twice the rate of a normal landing.
When a normally operating aircraft has hit terrain, it has usually occurred in bad weather or darkness, and after pilot confusion about the precise location or flight path, according to safety experts. Ground-collision warning devices have practically eliminated such fatal events in the U.S., Europe and other advanced aviation regions, though they remain a major problem in developing countries. Such crashes are rarely triggered during the cruise portion of flights.
In Tuesday’s crash, there was no hint of bad weather or disorientation, and the plane had just reached cruising altitude when it began to descend.
There was no indication the plane stalled as has happened in other crashes, causing the plane to fall from the sky. When jets stall or pilots get into unusual maneuvers and lose control of airliners, the result typically is a rapid dive. Lack of manual flying skills and misguided reliance on automation usually are primary culprits. When Air France Flight 447 plunged from the sky into the Atlantic in 2009 killing all 228 on board, the jet fell at times at three times the rate of descent experienced on the Germanwings plane.
This time investigators hope the black box recorders will reveal whether the pilots were conscious, how specific systems were operating and if the crew fought to save the jet. The recorders could also indicate whether the jet suffered a highly unusual, massive electrical fault that disabled some electrical equipment, preventing pilots from contacting ground controllers and leading to a crash.
French authorities said they weren’t ruling out any potential cause for the crash but didn’t appear to suspect foul play. The fact that prosecutors in Marseilles were assigned to investigate the crash—rather than the anti-terrorism unit based in Paris—indicated investigators were focused on other theories.
Flight 9525 took off from Barcelona at 10:01 a.m. local time with at least 67 German passengers and a large group of Spaniards on board. French officials said several Turks may also have been on the plane.
The plane had undergone regular maintenance the day before and had its last major safety check in 2013, according to Germanwings.
In the cockpit, the pilot had over 10 years of experience, and had clocked more than 6,000 hours flying Airbus jets.
The crash is the first fatal accident for Lufthansa since 1993, when two people died in an Airbus A320 crash in Poland, according to the Aviation Safety Network, an accident-tracking site run by the not-for-profit Flight Safety Foundation.
The Airbus A320 is the plane maker’s most popular model, with more than 3,600 in service and more than 4,700 sold. The A320 family of planes suffers about 0.08 crashes per million flights, according to website AirSafe.com—about the same as for equivalent Boeing narrowbodies, it said.
Airbus said the plane was delivered in 1991 to Lufthansa and had logged around 58,300 flight hours in some 46,700 flights.
The plane was transferred to Germanwings last year, the carrier said.
“It is a tragic and very sad day for Germanwings and the entire Lufthansa family,” Chief Executive Thomas Winkelmann said.
—Stacy Meichtry and Andy Pasztor contributed to this article.
Write to William Horobin at William.Horobin@wsj.com