A young German co-pilot barricaded himself alone in the cockpit of Germanwings flight 9525 and apparently set it on course to crash into an Alpine mountain, killing all 150 people on board including himself, French prosecutors said on 26 March.
They offered no motive for why Andreas Lubitz, 27, would take the controls of the Airbus A320, lock the captain out of the cockpit and deliberately set it veering down from cruising altitude at 3,000ft per minute.
German police searched his home for evidence that might offer some explanation for what was behind the 24 March crash in the French Alps.
The scenario stunned the aviation world. Within hours of the prosecutors' announcement, several airlines responded by immediately changing their rules to require a second crew member to be in the cockpit at all times. That is already compulsory in the United States but not in Europe.
"The failing here is only allowing one person in the cockpit at any one stage," aviation security expert Norman Shanks said.
"I would say that it's an absolute rule that no matter what is happening, there should always be two people in the cockpit, or alternatively, if you do need to leave, then there should be someone else who is trained to fly or has a rudimentary knowledge that can come in and stand in. So there is always occupancy of two people," he added.
Under German aviation law, pilots may temporarily leave the cockpit at certain times and in certain circumstances, such as while the aircraft is cruising.
Cockpit doors can be opened from the outside with a code, in line with regulations introduced after the 11 September 2001 attacks in the United States, but the code can be overridden from inside the cockpit, making the door impenetrable.
"The locked cockpit door and the strengthened cockpit doors came in as a consequence of 9/11 to prevent unauthorised people – hijackers – getting into the cockpit, taking the aircraft over and using it as an item of mass destruction. So they introduced strengthened doors, locks on doors – there is a keypad, which the occupants, the flight crew, would be aware of the right code so they can get back into the cockpit. There would also be part of that process, a way in which you could identify to the person inside that you are pressing that under duress, so that would allow the person in the cockpit to push the button to deny opening of the door," Shanks said.
Canada said it would now require two people to be present in the cockpit on all its airlines. EasyJet, Norwegian Air Shuttle and Air Berlin were among other carriers that swiftly announced such policies.
Among those that didn't was Germanwings parent company Lufthansa, whose CEO said he thought it was unnecessary. But the airline came under swift pressure on social media to make such a change and later said it would discuss it with others in the industry.
French and German officials said there was no indication Lubitz was a terrorist but offered no rival theories to explain his actions. Acquaintances described him as an affable young man who had given no sign of harmful intent.
Lubitz acted "for a reason we cannot fathom right now but which looks like intent to destroy this aircraft", Marseille prosecutor Brice Robin said.