We know that Germanwings Flight 4U 9525 was flown into a mountain by co-pilot Andreas Lubitz, 27, after he locked the plane's pilot out of the cockpit.
A German criminal investigation into the crash concluded in January that Lubitz bore sole responsibility for crashing the jet.
The final accident report by the French authorities answered many questions - but some are still a mystery.
Why did Lubitz do this?
Investigators ruled out terrorism early on and their position did not change when it was revealed that they believed Lubitz had deliberately crashed the aircraft.
That was upheld in the final report, which found that he had been suffering from psychiatric issues he had hidden from his colleagues.
The co-pilot did not behave in a peculiar or extreme manner and people who knew him expressed shock over the event.
"He's not the type of guy who would try and kill other people - absolutely not," a neighbour said.
"He was really just a very normal, not very remarkable, nice young man," said Klaus Radke, the president of the club near Montabaur in Germany where Lubitz had learned to fly.
Lubitz had battled depression before, suspending his training for medical reasons in 2008.
The final crash investigation report detailed his medical history and showed he had started to suffer from a "severe depressive episode without psychotic symptoms".
During that period, he considered suicide and "made several 'no suicide pacts' with his treating psychiatrist and was hospitalised", the report says.
In April 2009, his medical certificate was not revalidated "due to depression and the taking of medication to treat it".
In July that year, he was declared "entirely healthy" and given a one-year medical certificate, with a note that it must be renewed each year.
It was renewed annually without incident until December 2014.
It was then that Lubitz consulted doctors about "vision problems and sleep disorders" but eye specialists could not find a physical cause.
Instead, investigators say, he was diagnosed with a "psychosomatic disorder" - a physical illness caused by a mental factor.
In February 2015, he was given a sick certificate for eight days, which he did not forward to the airline.
Over the next month, he would be issued several more medical certificates, none of which his employer saw; was prescribed insomnia and antidepressant medication; and was referred "for psychiatric hospital treatment due to a possible psychosis".
Traces of the medications were found in his remains during a post-mortem toxicology exam.
"On the day of the accident, the pilot was still suffering from a psychiatric disorder, which was possibly a psychotic depressive episode, and was taking psychotropic medication," the report found.
"This made him unfit to fly."
Prosecutors in Duesseldorf at the time of the crash found torn-up sick notes, including for the day of the crash.
In April 2015, German prosecutors said Lubitz had researched "ways to commit suicide" on his tablet at home in the week before the crash. Another of his internet searches was for "cockpit doors and their security provisions".
The German tabloid paper Bild said (in German) it had seen documents that said he was suffering from a "personal life crisis", having recently broken up with a girlfriend.
But even when we have a fuller picture of Lubitz's state of mind before the crash, some of his actions may remain a mystery forever.
For example, after he had decided to kill himself and everyone else on board the plane, why did he set the aircraft on a controlled, rather than a sudden, descent?
Could depression really explain Lubitz's actions?
Depression is a serious illness, and it affects everyone differently. People experiencing depression can reach such a state of alienation that they risk taking their lives - but the vast majority of people with depression would never harm anyone other than themselves.
Writing in the Times, Jennifer Wild, a psychologist at the University of Oxford, has said that the irrational thinking evident from Lubitz's actions in the cockpit may have been triggered by depression, possibly caused by a recent traumatic episode. "Perhaps in his state he did not consider the consequences of his actions, or perhaps he did not care because he was consumed with ending his life," she wrote, adding that drug-taking or a bout of extreme anger might also explain his actions.
The black box recording of Lubitz's calm breathing as he pressed the controls to send the aeroplane hurtling to the ground may lead some to believe he was a psychopath but Dr Wild notes that psychopaths generally avoid harming themselves.
Sir Simon Wessely, president of the Royal College of Psychiatrists, said that millions of people in Britain suffered from depression, which is a treatable illness. He told the BBC that this included pilots who "resume flying, perfectly safely, for maybe tens of years afterwards".
The fact that Lubitz appears to have been signed off as sick raises the question of whether there was - or should have been - contact between his doctors and the airline.
Medical practitioners take an oath to maintain patient confidentiality but some US states have expanded doctors' duty of care to the community at large. This allows them to warn third parties if they think it necessary.
Do aviation authorities need to improve psychological screening?
In Germany, trainee pilots are subjected to medical examinations which include oral psychological assessments. There is some debate as to whether such assessments are thorough enough. One question an examiner might ask a pilot is, "Do you have a butterfly collection at home?", the Suddeitsche Zeitung reported (in German).
At the time of the crash, Lufthansa said that after they hired pilots they did not subject them to regular psychological examinations. This is obligatory in other countries, such as the US and UK.
Airline staff are encouraged to report strange behaviour on the part of their colleagues but a former pilot told the BBC that peer pressure could act as a brake on this process. And Carsten Spohr, Lufthansa's chief, admitted: "All the safety nets we are all so proud of here have not worked in this case."
But Mr Spohr also said: "No matter your safety regulations, no matter how high you set the bar, and we have incredibly high standards, there is no way to rule out such an event. This is an awful one-off event."
Aircraft-assisted pilot suicides are rare. A 2014 study by the Federal Aviation Authority identified eight such instances in the US between 2003 and 2012, accounting for 0.29% of all fatal aviation aircraft accidents. All the pilots involved had been medically screened and none had demonstrated mental disorder, depression or suicidal thoughts.
Is it true that the airline could have taken control of the plane from the ground?
Some people are asking why there is no system for wresting control of a plane from a control tower. In fact, such a system does exist, reports the Daily Mail, but it is not being used. In 2006, Boeing was awarded a patent for an "uninterrupted autopilot system" with its own power supply that could be activated by those on board a plane or on the ground. However, safety concerns - including the possibility that such a system could be hacked - have prevented its roll-out.
The crash also raises questions about the cockpit door mechanism which Lubitz used to keep the pilot out. The system, which allows a pilot to override the coded entry mechanism on the outside of the door, was designed in the event of a terrorist emergency.
Airlines are going to have to balance those concerns against the possibility that individuals like Lubitz might decide to do harm, said the BBC's transport correspondent Richard Westcott.
Can airlines prevent this from happening again?
Lubitz was able to fly the plane into the ground without interruption since he seized the opportunity to lock the cockpit doors while the plane's pilot was going to the toilet.
Some airlines enforce a "rule of two" - that there should never be fewer than two people left in the cockpit. For these airlines, a member of the cabin crew enters the cockpit when appropriate to ensure this rule is followed.
After the Germanwings crash, several airlines, including Lufthansa, announced they were adopting this rule.
The European Aviation Safety Agency has recommended that at least two people be present in the cockpit at all times. China's aviation authority has also introduced the rule and the UK's Civil Aviation Authority has contacted all UK airlines "to require them to review all relevant procedures".
Where does all this all leave relatives of those on board Flight 4U 9525?
What relatives of plane crash victims need most are answers, experts say. But the more we learn about Lubitz, the more heartbreaking the Germanwings crash seems.
Evidence from research suggests that trauma inflicted by other humans on purpose is more difficult to come to terms with than natural disasters, clinical psychologist Roderick Orner told the BBC.
Acts of violence are also harder to process than accidents due to negligence or mechanical failure. "Violence is often experienced as an attack on human integrity," says psychiatrist Lars Weiseth. "This aspect causes increasing anger, loss of trust in companies and people, and sharply increases the risk to mental health."
The fact that Lubitz seemingly had no agenda for his actions does not necessarily make things easier, since the deaths could seem meaningless. If he had been a terrorist, Prof Weiseth says, relatives might be able to see their family members as "involuntary participants in an important struggle for democratic values".