The co-pilot who deliberately crashed a plane in the French Alps with the loss of 150 lives had a history of depression, it has been reported.
The focus on Andreas Lubitz's mental health comes after a French prosecutor concluded the 27-year-old had deliberately flown the Germanwings Airbus 320 into the mountainside killing all those on board.
German prosecutors have said there are indications the co-pilot concealed an illness from his employer, hiding a sick note for the day of the crash.
However, they did not specify the nature of the illness.
Matthias Gebauer, chief correspondent for the online edition of German newspaper Der Spiegel, tweeted: "Schoolmates of co-pilot who crashed tell German reporters he took six-months break from flight training in 2009 due to burnout-syndrome."
The head of Lufthansa, the budget airline's parent company, has already admitted Lubitz had taken the lengthy break from training.
While chief executive Carsten Spohr did not give a reason for this interruption, German media reported he was suffering from "burnout or depression".
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Mr Spohr said: "I cannot tell you anything about the reasons of this interruption, but anybody who interrupts the training has to do a lot of tests so the competence and fitness would be checked again."
According to Lufthansa, Germanwings pilots undergo medical tests once a year.
However, they are only required to undergo psychological tests once, before they are accepted as pilots.
Lubitz also underwent a regular security check on 27 January and nothing untoward was found, the local government in Dusseldorf said.
Previous security checks in 2008 and 2010 also revealed no problems.
Lubitz had grown up dreaming of becoming a pilot, gaining his glider's licence after training with LSC Westerwald flying club in his hometown of Montabaur.
Club member Peter Ruecker recalled Mr Lubitz as "rather quiet but friendly" when he first joined the club as a teenager.
He added: "He was happy he had the job with Germanwings and he was doing well."
Lubitz had been employed as a flight attendant before training to be a pilot at the Lufthansa flight school in Bremen.
He also underwent training in Phoenix, Arizona.
Lubitz joined Germanwings in 2013 and had clocked up 630 flying hours before the disaster.
Lufthansa said he passed all the relevant examinations necessary to become a pilot and was deemed "100% airworthy".
Mr Lubitz had also been included by the US Federal Aviation Administration (FAA) on its database to show he had met or exceeded its pilot certification standards, which aim to "reduce pilot errors that lead to fatal crashes".
In Montabaur where Lubitz lived with his parents, neighbours reacted with disbelief when they heard of his involvement.
One man, who did not want to be named, said that he had known the pilot since childhood.
He told Sky News: "I cannot imagine that he has done it with intention.
"This does not fit in this picture I have of him. It is a very upright family, very helpful and I cannot understand what has happened.
"I knew the children when they were small boys."
Marseille prosecutor Brice Robin said Lubitz, who also had a flat in Dusseldorf, had never been flagged as a terrorist.
And when pressed over Mr Lubitz's religion, he said: "I don't think this is where this lies. I don't think we will get any answers there."
Although rare, there have been previous instances of suspected pilot suicide.
The most infamous likely - but still disputed - cases of pilot suicide was the 1997 Silk Air crash in Indonesia, in which 104 people died.
A US-led investigation concluded it had been caused deliberately, probably by the captain who had serious personal problems.
A Mozambique Airlines plane crash that killed 33 people in Namibia in 2013 is also believed to have been a case of pilot suicide.
