Investigators carry boxes from the apartment of Germanwings airliner jet co-pilot Andreas Lubitz. Photo / AP
Police investigating the Germanwings crash said they had made a 'significant discovery' at the home of pilot Andreas Lubitz, who deliberately ploughed the Airbus A320 into the French Alps.
Officers refused to reveal details of the potential breakthrough but said it was not a suicide note.
Speaking outside the flat on the outskirts of Dusseldorf, police said they had 'found something' that would now be taken for tests, adding it may be a 'clue' as to what happened to the doomed jet.
German detectives were also pictured carrying evidence from a £400,000 home in Montabaur, a town 40 miles from Bonn, that Lubitz is believed to have shared with his parents.
The 28-year-old is understood to have split his time between the two addresses.
The forensic find comes hours after it emerged that Lubitz was forced to postpone his pilot training in 2008 because of mental health problems, with a friend saying he was 'in depression'.
Read more:
• French Alps plane crash: Germanwings silent on locked-out pilot
• French Alps crash: Pilot locked out of cockpit
Airline bosses confirmed Lubitz had taken several months off work and had to retrain to join the firm, but insisted he was '100% fit to fly'.
The revelation will undoubtedly form a central part of the investigation and put pressure on the industry to explain whether enough was done to ensure Lubitz was fit to pilot a commercial aircraft.
It also emerged today that his parents only discovered that their son was a mass murderer after they had travelled to the crash scene with the victims' families.
His mother, a piano teacher, and father, a successful businessman, are expected to be questioned by police.
At an extraordinary press conference earlier, Marseille prosecutor Brice Robin gave a disturbing account of the cockpit voice recordings extracted from black box.
He said Lubitz locked his captain out after the senior officer left the flight deck.
At that point, Lubitz used the flight managing system to put the plane into a descent, something that can only be done manually - and deliberately.
He said: 'The intention was to destroy the plane. Death was instant. The plane hit the mountain at 700kmh (430mph).
'I don't think that the passengers realised what was happening until the last moments because on the recording you only hear the screams in the final seconds'.
Earlier in the flight, Mr Robin said Lubitz's responses were initially courteous, but became 'curt' when the captain began the mid-flight briefing on the planned landing of the plane.
The captain - named by local media as German father-of-two Patrick Sonderheimer - then left the cockpit but found himself locked out when he tried to re-enter.
Mr Robin said: 'We hear the pilot asking the co-pilot to take over and we hear the sound of a chair being pushed back and a door closing so we assume that the captain went to the toilet or something.
'So the co-pilot is on his own, and it is while he's on his own that the co-pilot is in charge of the plane and uses the flight management system to start the descent of the plane.
'At this altitude, this can only be done voluntarily. We hear several shouts from the captain asking to get in, speaking through the intercom system, but there's no answer from the cockpit.'
Audio from the recording captures Mr Sonderheimer furiously pounding on the door to no avail.
Mr Robin said Lubitz 'voluntarily' refused to open the door, adding that his breathing was normal throughout the final minutes of the flight.
He said: 'His breath was not of somebody who was struggling. He never said a single word. It was total silence in the cockpit for the ten past minutes. Nothing.'
Air Traffic Control at Marseille asked for a distress signal, but there is still no response.
He added: 'So the plane becomes a priority for a forced landing.
'Control asks other planes to contact this Airbus and no answer is forthcoming.
'There are alarm systems which indicate to all those on board the proximity of the ground. Then we hear noises of someone trying to break into the door.
'The door is reinforced according to international standards.'
Mr Robin went on: 'Just before final impact we hear the sound of a first impact. It's believed that the plane may have hit something before the final impact.
'There is no distress signal or Mayday signal. No answer was received despite numerous calls from the tower.'
Referring to Lubitz, Mr Robin said: 'He did this for a reason which we don't know why, but we can only deduct that he destroyed this plane.
'We have asked for information from the German investigation on both his profession and personal background'.
Mr Robin said he had no known links with terrorism, adding: 'There is no reason to suspect a terrorist attack.'
And asked whether he believed the crash that killed 150 people was the result of suicide, he said: 'People who commit suicide usually do so alone... I don't call it a suicide.'
Responding to revelations, Lufthansa CEO Carsten Spohr said all pilots undergo annual medical checks, but not special psychiatric assessments beyond training.
He added: 'He passed all medical exams, all checks. He was 100 per cent fit to fly without any restrictions.
'I am not a lawyer. I am the CEO of a big company. If one person takes 149 people with him to death, it is not suicide.'
Germanwings said: 'Not in our worst nightmare could we imagine something like this happening.'
However, it emerged today that Lubitz had to stop his pilot training in 2008 because he was depressed and suffering 'burnout'.
A schoolmate told the Frankfurter Allgemeine Zeitung that he had taken a break because of depression.
The woman said: 'Apparently he had burnout, he was in depression.'
Lubitz had just 600 hours of flying experience after joining Germanwings in 2013 straight from training.
He was, however, highly regarded, having won an award from the Federal Aviation Administration in 2013 for his outstanding flying skills.
He is understood to have shared his time between his parents' house in Montabaur, a town 40 miles from Bonn, and an address in Dusseldorf.
He was a member of the LSC Westerwald flying club in Montabaur.
His recently deleted Facebook page appeared to show him in a dark brown jacket posing in front of the Golden Gate Bridge in California. The page was wiped at some time in the past two days.
He lists his interests as aviation and music, including French DJ David Guetta.
In Montabaur, acquaintances today said Lubitz showed no signs of depression when they saw him last autumn when he renewed his glider pilot's license.
'He was happy he had the job with Germanwings and he was doing well,' said a member of the glider club, Peter Ruecker, who watched Lubitz learn to fly. 'He gave off a good feeling.'
Lubitz had obtained his glider pilot's license as a teenager and was accepted as a Lufthansa pilot trainee after finishing a tough German college preparatory school, Ruecker said.
He described Lubitz as a 'rather quiet' but friendly young man.
Klaus Radke, chairman of the flying club, told MailOnline that he flew at the club between the ages of 14 and 20.
He said he was a 'normal, open-minded person', adding: 'I can't believe that he has died.
'It was his dream to become a pilot and he had been extremely happy after fulfilling this dream'.
A black ribbon bearing the number of the doomed Germanwings flight he was aboard appeared on his flying club's website.
A tribute to him read: 'With great dismay, the members of the LSC Westerwald e.V have heard of the crash of Germanwings flight 4U9525.
'With horror, we acknowledge that among the dead is a longtime member of our association.
'Andreas died as First Officer on the tragic flight.
'As a teenager, Andreas joined our club to realise his dream of flying. He started as a glider student and made it to be a pilot on an Airbus A320.
'It was his dream fulfilled, the dream he so dearly for with his life. The members of the LSC Westerwald mourn Andreas and the other 149 victims of the disaster.
Our deepest sympathy goes out to the victims of all nationalities. We will not forget Andreas.'
The tribute was removed from the internet minutes after French prosecutors accused him of murder.
One colleague told Germany's Rhine newspaper: 'He was a fitness fanatic who jogged most mornings and evenings and you could often find him returning home from sports shops carrying health supplement bags.
'He was a friendly guy, who had a great deal of respect in the town, he was mostly engaged with the local flying club.'
Another woman identified only as Ulrika told Bavarian radio: 'Everybody is stunned in this town to learn the news, he grew up here, went to school here and came back most weekends from Dusseldorf when he wasn't flying.
'This is a great loss to us, and we are perplexed to think that he might have caused the deaths of so many people.'
Police were called out to his home in the Rhineland town as numerous groups of journalists descended on the town seeking information about him.
It is understood that his parents will be questioned as part of the investigation, which will undoubtedly focus on his mental health in the lead-up to the disaster.
The investigation is now a full-blown criminal enquiry following revelations of the argument on board.
The fate of the Germanwings plane has chilling similarities to that of LAM flight 470 which crashed in Namibia in November 2013, killing all 27 passengers and six crew.
Air crash investigators believe the Embraer 190 jet was flown into the ground by the captain after his co-pilot went to the toilet.
The jet's captain, Herminio dos Santos Fernandes was believed to have had serious personal problems at the time of his death.
When his co-pilot went to the toilet, flight data information recovered from the scene found that Fernandes manually changed the aircraft's altitude from 38,000 feet to almost 600 feet below ground level.
He also pushed the aircraft's throttles back to idle and selected the jet's maximum operating speed.
Distubringly, the cockpit voice recorder picked up the sound of the co-pilot pounding on the door in an attempt to regain access to the flight deck.
Four specialists from Interpol have joined senior French detectives trying to work out why Lubitz locked himself into the cockpit.
The Airbus A320 suddenly began a fatal eight-minute descent shortly after reaching cruising altitude.
No distress signal was sent and the crew failed to respond to desperate attempts at contact from ground control.
Interpol, the international criminal police organisation, today confirmed that it has sent a team of experts to assist with the enquiry at the request of the French enquiries.
Four Interpol officers will initially be based with a crisis cell being coordinated from Paris.
Jurgen Stock, head of Interpol, said: 'Interpol is committed to providing all the support required by countries hit by this tragic accident'.
The revelations came after audio files taken from the black box recorder had earlier suggested that one of the pilots was forced to try and smash down the door after being unable to enter the flight deck, according to a report in the New York Times.
Experienced pilots today told MailOnline that under normal conditions crew have an emergency access code to enter the cockpit through the locked door.
They can only be stopped from using it if whoever is inside the cockpit manually - and intentionally - disables it.
The revelation will heighten fears that suicide or a terror attack was the cause of the disaster.
Locks on cockpit doors were introduced throughout the world's airlines in the aftermath of 9/11 to keep terrorists from taking the controls in a hijacking.
Tony Newton, a Civil Aviation Authority examiner and commercial pilot with 20 years' experience of flying A320 aircraft, told MailOnline: 'This takes the whole thing off in a different direction.
'Blocking access requires a deliberate action on behalf of the pilot. It's a pretty dark thing to have happened.'
Mr Newton told MailOnline: 'When the cockpit door is locked, it is possible for the crew to punch in a code from the outside and gain access, unless the person in the cockpit over-rides it.
'The person in the cockpit can see them on CCTV trying to get in, and flick the switch to block their request. This is in case an undesirable person outside is trying to gain access.
'If the person in the cockpit doesn't want you coming in, you're not coming in.
'In the system used by the majority of airlines, if the pilot passes out in the cockpit, you can always get in.'
Mr Newton said there have been examples of pilots committing suicide by crashing their planes, notably the Egypt Air Crash in 1999 and the Silk Air disaster in 1997.
He added: 'And then there is obviously 9/11, which was for ideological reasons.
'It is almost certainly either ideological or the result of depression. It's too early to say for sure, but the options are narrowing.
'Outside of that, there would have to be significant new information for us to draw a different conclusion. It's a pretty dark thing to have happened.'
Cockpit recordings recovered from the crash site indicated one of the seats was pushed back and the door opened and closed.
An unnamed military investigator told the New York Times: 'The guy outside is knocking lightly on the door and there is no answer.
'And then he hits the door stronger and no answer. There is never an answer. You can hear he is trying to smash the door down.'
A source told AFP news agency that an alarm indicating the proximity to the ground could be heard before the moment of impact.
The recording has shed new light on the missing eight minutes from 10.31am when air traffic controllers lost contact with the pilots, who failed to send any distress signal.
Details from the first report submitted by the French to the German government revealed that at 10.31am, the 24-year-old Airbus A320 left its assigned altitude and began dropping towards the ground at a speed of 3,500ft per minute, before smashing into a ravine at 6,200ft.
The report said controllers tried three times on an assigned radio frequency to contact the pilots before switching to international emergency channels.
No one answered and a French Mirage fighter jet was scrambled.
The pilots have been the focus of the investigation from the outset, yet Germanwings' parent company Lufthansa refused for 48 hours to release their identities, not even their age or nationalities.
The firm only confirmed that the captain had 6,000 flying hours and been a Germanwings pilot since May 2014, having previously flown for Lufthansa and Condor.
Another aviation expert told AFP: 'If the pilots did not stop the airplane from flying into the mountains, it is because they were unconscious or dead, or they had decided to die, or they were forced to die.'
The cockpit door also has several other safety features in case of a sudden decompression which will cause the door to open.
This could rule out a scenario where the pilot locked the door as a safety precaution because of a structural failure in the cockpit which caused a drop in oxygen levels.
A spokesman for the BEA, France's accident investigation office, would not comment on the revelations last night.
Earlier in the day, BEA spokesman Remi Jouty confirmed voices could be heard on the damaged voice recorder, which covered the flight 'from departure to crash'.
But they warned that it could take 'days, weeks and even months' before analysts were able to determine exactly what it being said or what the noises are.
The cockpit voice recorder is designed to store two hours of conversation and withstand impacts of as much as 3,400 times the force of gravity.
The breakthrough in the investigation came as bereaved families began arriving from Spain and Germany near the remote mountainous crash site.
All 150 people on board, including three Britons, died when the Germanwings flight 4U 9525 from Barcelona to Dusseldorf smashed into the mountains after an eight-minute descent.
The Airbus A320 sent no distress signal and the crew failed to respond to ground control's desperate attempts to make contact.
The cockpit recording showed the pilots speaking normally in German at the start of the flight, the source said, adding that it could not be determined if it was the captain or the first officer who left the cockpit.
In a statement overnight, Germanwings said that 'at the moment, we do not have information from competent authorities to confirm this story.'
'We are doing everything to get the most information possible and we are not engaging in speculation.'
Authorities have said they have no explanation as yet for the tragedy, but said that the plane was still flying when it crashed into the mountain and did not explode mid-air.
On Wednesday, the head of France's BEA crash investigation agency head Remi Jouty told reporters he still had 'not the slightest explanation' for the tragedy at this stage.
'It is inexplicable,' Lufthansa chief executive Carsten Spohr said in Frankfurt.
'The plane was in perfect condition and the two pilots were experienced.'
Investigators are still hunting for the second black box, which would reveal technical flight data.
President Francois Hollande said on Wednesday the casing of this box had been found but not the device itself.
France's interior minister has said the probe is not focusing on a possible terror attack.
The Mental Health Foundation's free resource and information service (09 623 4812) will refer callers to some of the helplines below:
Lifeline -- 0800 543 354
Depression Helpline (8am to midnight) -- 0800 111 757
Healthline -- 0800 611 116
Samaritans -- 0800 726 666 (for callers from the Lower North Island, Christchurch and West Coast) or 0800 211 211/(04) 473 9739 (for callers from all other regions)
Suicide Crisis Helpline (aimed at those in distress, or those who are concerned about the wellbeing of someone else) -- 0508 828 865
Youthline -- 0800 376 633, free text 234 or email talk@youthline.co.nz
- Daily Mail
Residential areas in both countries have suffered from deadly strikes in the conflict.