DÜSSELDORF, Germany — The copilot of the Germanwings flight locked himself in the cockpit and, with apparent cool precision, deliberately slammed into snow-capped mountains, a French prosecutor said Thursday, in a stunning twist to a tragedy that killed all 150 people aboard.
The conclusions, based on cockpit flight recordings recovered from the wreckage, abruptly shifted the investigation into a realm that Germany’s chancellor called ‘‘incomprehensible:’’ an intentional, eight-minute descent Tuesday that ended with the A320 jet disintegrating in the rocks and ravines of the French Alps.
‘‘This action can only be done deliberately,’’ the Marseille, France-based prosecutor, Brice Robin, told reporters.
His account offered a chilling and calculating scenario: the plane dropping in a steep, but steady, rate that did not appear to startle most passengers until it was clear it was about to hit the snow-bound peaks in southern France.
‘‘The screams are not heard until the very last moments,’’ Robin said.
Robin also said the flight recorder showed the copilot — identified as Andreas Lubitz — did not say a word after the captain left the cockpit. All that was heard was his breathing until the crash.
‘‘It was absolute silence in the cockpit,’’ Robin said, despite reports that the audio had the sounds of someone — apparently the pilot — banging on the door.
Robin said the copilot had no known links to suspected terrorist groups but noted the investigation remained wide open.
‘‘People who commit suicide usually do so alone . . . I don’t call it a suicide,’’ he said.
A ‘‘new, simply incomprehensible, dimension,’’ German Chancellor Angela Merkel said after learning of the French claims.
‘‘Something like this goes beyond anything we can imagine,’’ she added.
Lubitz, 27, had been flying with Germanwings since September 2013 and had flown 630 hours, said Lufthansa, the parent company of the budget carrier. In his native village of Montabaur, about 50 miles northwest of Frankfurt, Lubitz had been a member of a local aviation club since he was a teenager.
Germany’s interior minister, Thomas de Maiziere, said checks on domestic police and intelligence databases on the day of the crash turned up no red flags concerning Lubitz.
Like nearly all pilots in the Lufthansa airline group, he did part of his training at the carrier’s facility outside Phoenix — selected by many aviation schools for the area’s arid and clear weather.
Lubitz’s Facebook page included a photo of him smiling in a relaxed pose with the Golden Gate Bridge in the background.
Lubitz once interrupted his training. Lufthansa could not make public the reasons for the break, but considered Lubitz ‘‘fit to fly’’ on Tuesday, said the carrier’s chief executive, Carsten Spohr.
‘‘We at Lufthansa are speechless,’’ he told reporters in Cologne, Germany.
In response to questions about his views of the possible motives, Spohr added: ‘‘When someone kills himself and 149 others . . . it is not a suicide.’’
Spohr said all normal procedures were followed when the pilot left the cockpit — waiting until the flight was at cruising altitude and with no weather problems foreseen.
US carriers require two people in the cockpit, bringing in a member of the flight crew if a pilot needs to leave, but it is not required by Lufthansa and many other airlines. Normally, crew members step out for a bathroom break.
Canada and Germany’s biggest airlines, including Lufthansa and Air Berlin, as well as low-cost European carriers easyJet and Norwegian Air Shuttle announced new rules Thursday requiring two crew members to always be present.
Some experts said even two isn’t enough, and called for rules to require three.
‘‘The flight deck is capable of accommodating three pilots and there shouldn’t ever be a situation where there is only one person in the cockpit,’’ said James Hall, a former chairman of the National Transportation Safety Board, referring to the ‘‘jump seats’’ all airliners are equipped with.
Others questioned the wisdom of sealing off the cockpit at all.
‘‘The knee-jerk reaction to the events of 9/11 with the ill-thought reinforced cockpit door has had catastrophic consequences,’’ said Philip Baum, London-based editor of the trade magazine Aviation Security International.
Spohr described the cockpit exchanges between the pilot and Lubitz as normal, even ‘‘cheerful’’ at times.
‘‘You are in control,’’ the pilot said to Lubitz as he left, Spohr recounted.
The audio recording later carries the increasingly frantic sounds of someone banging at the cockpit door.
The captain of the plane had more than 6,000 hours of flying time and had been a Germanwings pilot since May 2014, having previously flown for Lufthansa and Condor, news reports say.
An Airbus training video shows that the cockpit door of the A320 has safeguards in case one pilot becomes incapacitated inside while the other remains outside, or if both pilots inside lose consciousness.
If there is no response, a member of the flight crew can tap in an emergency code. If there is still no response, the door opens automatically. If a person has been denied access, the door remains locked for five minutes, according to the training video.But the Lufthansa chief executive said there are ways to override the system and fully lock the cockpit.
‘‘No messages of distress or emergency were sent from the cockpit and no answers were given to the calls of the air traffic controllers,’’ Robin said.
‘‘The copilot deliberately refrained from opening to the pilot, and activated the device leading to the loss of altitude of the plane,’’ he added. ‘‘The reason why the copilot chose to do this is unknown so far. His act can be seen as a will to destroy the plane.’’
Among the dead — 144 passengers and six crew — were three Americans, the State Department said. The other victims of the crash were mainly from Germany and Spain.
‘‘The plane was cruising at 38,000 feet — planes don’t crash in cruise,’’ said Anthony Davis, a London-based aviation specialist. ‘‘They crash in takeoff or landing or they have engine failure, but it’s very unusual anything should happen at that altitude.’’
More on the French Alps crash:
Material from the Associated Press was used in this report.