Andreas Lubitz never said a word; never gave any clue as to what must have been racing through his mind.
Sitting at the controls of Germanwings Flight 9525, he reached down to the central console and, with his left hand, flicked the switch to lock the plane's captain out of the cabin. He then leant forward and reset the aircraft's altitude, turning the dial of the altimeter to 100ft above sea level, the lowest possible setting. With the plane still on autopilot, Lubitz then sat back in his co pilot's chair and did precisely nothing.
He may have shut his eyes or he may have watched out the cockpit window as the Airbus 320 hurtled into a mountain in the Alps at a speed of almost 550 miles an hour, killing all on board. Throughout the eight-minute descent, as the frantic captain made increasingly desperate efforts to smash his way into the cockpit, Lubitz remained calm and unmoved.
On the black box voice recorder, recovered at the scene, all that can be heard coming from Lubitz's mouth is his quiet, regular breathing. He offered no words of explanation as to why he had embarked on such a murderous course of events. There was no muttering, no ranting from a mass killer: just gentle breathing, until that was drowned out first by the banging of a crowbar on the cockpit door as Captain Patrick Sonderheimer attempted to regain control and then, in the final few seconds before impact, by the screams of the passengers as they realised their fate.
On board for the short-haul flight from Barcelona to Dusseldorf were 144 passengers and six crew. Among them were 16 children and two teachers from the Joseph König Gymnasium in Haltern am See, who were returning to Germany after a week-long foreign exchange in Spain.
Andreas Lubitz's remains are scattered across the mountains, along with those of his victims. What turned him from an apparently successful pilot, with a promising career ahead of him, into a mass murderer at the age of just 27 is the subject of inquiries by authorities across a number of jurisdictions.
From a variety of sources - official and unofficial - we now know that Lubitz suffered from depression, had had some kind of "burnout", and was being treated for an unspecified illness that should have prevented him from flying on that fateful day.
He was a man apparently in torment. One former girlfriend claimed that Lubitz had become increasingly volatile and had talked grandiosely of doing "something that will change the whole system and then all will know my name and remember it".
The starting point for investigators trying to understand Lubitz's mindset will be his home town, Montabaur, in the Rhine, half-way between Cologne and Frankfurt. Yesterday afternoon, his father Günter (54) and mother (still not named) were due to be interviewed by the police. Mr and Mrs Lubitz had travelled to France with other grieving relatives, only to arrive there and learn their son had been identified as the killer.
He lived with his parents in an affluent suburb of the town, in a house worth half a million euro. His father is a wealthy businessman; his mother a piano teacher who plays organ in the local church. The couple had two sons, Andreas was the older.
He was, from a very young age, obsessed with flying. "He was a real fanatic," said a pal. "His room was plastered with pictures of planes and the Lufthansa logo everywhere."
At the age of 14, Lubitz joined the local flying club, Luftorts Club Westerwald, a short drive from the family home. After a year of lessons under dual controls, he was able to fly a glider on his own.
Lubitz applied for a job at Lufthansa, the German national airline that also owns the budget airline Germanwings. Lufthansa is reckoned among pilots to be just about the best carrier to work for. Lubitz passed a number of rigorous assessments - including psychometric testing of a candidate's ability to work under pressure and handle stress - to be accepted on its pilot training programme. But despite his success in getting into Lufthansa's flight school in 2008, there were serious signs emerging that Lubitz was a deeply troubled young man.
In 2009, part-way through his training, he suffered a "serious depressive episode" and began to receive some form of counselling or psychiatric treatment, according to the German newspaper Bild.
Another source said: "During his training at Lufthansa flight school, Andreas was listed as unsuitable for flight duties, because he spent one and a half years in psychological treatment, and so he had to repeat courses." Der Spiegel reported that he had suffered "burnout syndrome".
The airline has been more circumspect. Lufthansa CEO Carsten Spohr said: "Six years ago there was a lengthy interruption in his training. After he was cleared again he resumed training. He passed all the subsequent tests and checks with flying colours."
He added: "He took a several months break for reasons I do not know. Then he had to do the tests again." In comments that will come back to haunt him, Mr Spohr had insisted Lubitz was "100 per cent fit to fly without any limitations". That claim has turned out to be false.
On qualifying, Lubitz did not land a job as a pilot immediately and worked as a cabin steward until a vacancy arose. It earned him the nickname 'Tomato Andi', possibly because one of his roles was to hand out tomato juice. For Lubitz, a young man wanting to fly jets, to find himself working as a "trolley dolly" would have been demeaning.
In September 2013, a vacancy finally appeared for a pilot, albeit with Lufthansa's less glamorous sister airline, Germanwings, which flies short-haul budget flights across Europe.
For Lubitz, recovering from depression and a long stint as an air steward, the realisation that he had not landed the dream job he had hankered after ever since childhood might have hit him hard.
If it did, Lubitz was keeping his feelings to himself. At a barbecue at the gliding club in Montabaur last summer, Lubitz appeared to have it all. "He seemed normal; proud of his job after so much training. He seemed happy," recalled Klaus Radker.
By then, Lubitz had moved into a flat in Dusseldorf, where Germanwings has a hub airport. It is thought the partner whom he took to the barbecue lived with him. Nobody at the apartments last week would talk about their neighbour. Police had sealed off the flat and there was no evidence of a girlfriend living there, although the nameplate "Goldbach" sits with his at the entrance.
Bild newspaper said that, at the time of the crash, Lubitz had broken up with her. He was said to be suffering from a "personal life crisis".
A previous girlfriend, known only as Maria, gave an astonishing account of Lubitz as a man fighting his demons as their five-month relationship soured. She was an airline hostess, and they had met on a flight, sparking a tempestuous romance that comprised fleeting assignations in hotel rooms around Europe and Germany.
Maria said she thought he had crashed because he knew his illness would prevent him progressing to Lufthansa jets.
"He did it because he realised that because of his health problems his big dream of a job with Lufthansa, a job as captain and as a long-haul pilot, was as good as impossible," she said.
They split up when she felt unable to deal with his growing problems and his increasingly volatile temper any more. "During conversations he'd suddenly throw a tantrum and scream at me. I was afraid. He even once locked me in the bathroom for a long time." At night he would wake up in a sweat, screaming in terror "We're going down", she said, and he also threatened a spectacular gesture to "make everyone remember" him.
Just weeks before Tuesday's crash, Lubitz had bought two Audis from a local dealership, one for him and one for his now ex-partner, the teacher, in a desperate bid to win her back. Only one of the cars was delivered, suggesting that she had declined his gift.
Police are investigating claims that he went on a spending spree in the weeks before the crash, despite his worries about money. Fabrizio Poli, an aviation expert with his own private jet company, said Lubitz would have been earning in the region of €130,000 as a Germanwings pilot, despite his junior ranking. But a large slice of that would have been repaid to Lufthansa to pay for training.
By the time of the crash, Lubitz, a fitness fanatic, was clearly struggling with an as yet unidentified illness, and one that he kept secret from his employers. If he couldn't fly, money worries would inevitably have been compounded. Certainly, on the day of the crash, Lubitz had been given a sick note excusing him from going to work. Instead of handing it in, he ripped it up. Five other medical notes were also found torn up.
"Documents with medical contents were confiscated that point towards an existing illness and corresponding treatment by doctors," said the prosecutors' office in Dusseldorf. "The fact that there are sick notes saying he was unable to work that were found torn up, which were recent and even from the day of the crime, supports the assumption based on the preliminary examination that the deceased hid his illness from his employer and his professional colleagues."
What that illness was is unclear. Dusseldorf University Hospital, where he had been treated in February and most recently on March 10, said in a statement that Lubitz had gone to the hospital for "diagnostic evaluation".
It declined to provide details about his condition, but denied German media reports that it had treated the pilot for depression.
Yesterday Die Welt, quoting German police, reported that investigators had found evidence that Lubitz had a "serious psychosomatic illness" and that medication for the treatment of severe burnout syndrome had been discovered in Lubitz's flat. The syndrome is linked to high levels of stress in the workplace and can cause suicidal tendencies.
But The New York Times offered an alternative explanation for his treatments in February and March, suggesting that Lubitz was suffering from "vision problems". Deteriorating eyesight would have cut short his career and, for a man who had only ever wanted to be a pilot, such a condition would have been devastating.
His medical files have been passed to the authorities examining the crash. They will try to work out what was going on when he took control of Flight 4U9525 and flew it into a mountain.
Lubitz had to wait for the plane's captain to leave the cockpit for a toilet break, an event that had happened routinely on previous flights. But it would indicate that the decision may not have been pre-planned, because had Capt Sonderheimer not needed to answer a call of nature then Lubitz would not have been left alone to crash the plane into an area of the French Alps he knew well from days on gliding holidays there.
As he sat in the pilot's seat, in sole charge of a multimillion-pound aircraft, responsible for the lives of 149 other people, Lubitz had made a decision to damn them all. He wanted to die and here was his opportunity.
The question for that authorities trying to understand his actions is, was he so caught up in his own world, and his own misery, that he didn't care what happened to the lives of others - or did he deliberately make a statement declaring his own importance by killing them too?
The silence on the black box recorder, and the seeming absence of a suicide note, leaves those questions unanswered.
But in those eight minutes of silence, Lubitz revealed himself to the world as a calm, cold-blooded killer, who will go down in history - perhaps just as he wanted - as a "madman" who destroyed the lives of 149 innocent individuals.