The co-pilot of the Germanwings flight that crashed in the French Alps appears to have deliberately flown the aircraft into a mountainside, killing 150 people, after locking the captain out of the cockpit, French prosecutors said.

Audio files from the last minutes on the flight-deck of the Barcelona-Dusseldorf flight show the co-pilot took the aircraft into a descent after the captain left and denied him re-entry, prosecutor Brice Robin said at a press conference in Marseille. The co-pilot could be heard breathing and remained otherwise silent right until the plane slammed into a slope at full speed.

The findings suggest that the destruction of the Airbus A320 aircraft, the worst-ever aviation accident for Deutsche Lufthansa AG and its Germanwings unit, was deliberate rather than due to a technical fault. The co-pilot was named by Robin as Andreas Lubitz, a 28-year-old German citizen who Lufthansa said had been deemed completely fit to fly following tests.

“The most plausible interpretation is that the co-pilot voluntarily refused to open the door for the captain and activated the button controlling the plane’s altitude for a reason we don’t know,” Robin said. The action “can be interpreted as a desire to destroy the plane,” he added.

Pulverized

The crash on Tuesday had mystified investigators because the plane was flying in normal daylight conditions and had undergone all required maintenance checks. While the plane’s voice recorder was found and provided the audio evidence, a second black-box, which stores data on the plane’s performance, has not yet been recovered from the field of debris, Robin said.

Salvage crews on site have begun recovering bodies from the wreck, which was largely pulverized because of the speed of an impact that Robin put at 435 miles an hour. It will be the week after next before all of the bodies are recovered from the steep terrain, he said.

Lufthansa CEO Carsten Spohr said Lubitz had passed all medical tests and checks and had started training in 2008, both in Bremen in northern Germany and in Phoenix, Ariz., before joining the airline in late 2013. Cockpit personnel undergo thorough examination, said Spohr, calling the incident a “tragic, isolated case.”

The co-pilot had interrupted his pilot training for several months, though Spohr said he couldn’t give a reason. Pilots undergo medical tests once a year, and a psychological examination occurs at the beginning of their career, Spohr said.

Fortified Door

Germanwings said it cannot comment further because the investigation is a matter for the authorities. France’s BEA air-accident investigator is leading the probe into the tragedy, which killed 144 passengers and six crew.

Lufthansa, which operated the A320 before it was handed to Germanwings in early 2014, said the cockpit had a fortified door with video surveillance to prevent unauthorized entry, a measure that became mandatory after the 9/11 terror attacks. While pilots have a security code that lets them open the door from the outside, the person in the cockpit can still deny access.

Some airlines require two people on the flight-deck when one pilot steps out, making it necessary for cabin crew to stand in, though Lufthansa said there’s no uniform rule on this. Norwegian Air Shuttle ASA said Thursday that it would adopt that rule once the switch has been approved by local regulators.

Richard Healing, a former member of the U.S. National Transportation Safety Board, said the events disclosed Thursday leave some room for alternative interpretation.

‘Incapacitated’

“You can’t rule out the possibility at this point of the pilot who was remaining in the cockpit becoming incapacitated simultaneous with when the other pilot walked out,” he said. “It’s more likely that the pilot may have done something intentionally, but you can’t conclude that until the flight data recorder is recovered.”

The recorder should help determine what pilot inputs were made, including whether the autopilot was switched off and how the altitude selector was programmed, as well as whether any flight systems failed, said Healing, who now runs R Cubed Consulting LLC in Washington.

The captain was “very experienced” and had flown for charter carrier Condor and Lufthansa’s main brand for about 10 years before joining Germanwings in May 2014, the budget airline said. He had logged more than 6,000 flight hours.

Germanwings declined to provide personal details or the ages of the pilots, adding that both were trained “according to Lufthansa standards.”

Lubitz came from Montabaur, about an hour north of Frankfurt, according to the local mayor’s office, and had been part of the local aviation club, which said on its website that it was “horrified” to learn that a member was among the dead.