Investigators believe that Lubitz, 27, deliberately flew an Airbus A320 with 150 people on board into a remote corner of the French Alps last week, provoking a search for answers that is focusing on his health, and his mental health in particular.
The picture emerging of Lubitz is one of a man haunted, whose ambition to fly brought him both pleasure and torment. Authorities have found doctors' sick notes stating he was unfit for work, including on the day of the crash. Germany's Bild newspaper quoted an interview with an ex-girlfriend of Lubitz's who described a man who suffered from both vivid nightmares and delusions of grandeur.
"At night, he woke up and screamed: 'We're going down!' because he had nightmares," the former girlfriend told Bild. "He knew how to hide from other people what was really going on with him."
She added that last year he had warned, "One day I will do something that will change the whole system, and then everybody will know my name and remember it."
Bild and the New York Times reported that Lubitz was seeking treatment for vision problems, though it remained unclear whether the issue was real or perhaps psychosomatic. Such concerns could have led Lubitz to worry that he would lose his medical clearance to fly.
Here in his hometown in southwestern Germany, a city of 13,000 dotted with traditional houses and well-tended lawns, the mood shifted between denial and sorrow over its local boy made good. Yet Lubitz was hardly a forceful presence here, and those who knew him described him as friendly, even bland - a non-memorable man who called Guten Tag to neighbours on his morning runs but was otherwise quiet and reserved.
"He was inconspicuous, normal, nice," said Michael Dietrich, the pastor at the Luther Church in Montabaur who taught Lubitz's confirmation class.
But on the day Lubitz appeared to fly that Airbus into a chilly French mountainside, he was hiding a potentially deadly secret: a chronic medical condition that a doctor had determined was serious enough to keep him out of the sky.
Authorities would not reveal the exact nature of Lubitz's illness. But an official from the German prosecutor's office in Duesseldorf said earlier that the doctors' notes were related to a "long-lasting condition". Asked whether they were also related to psychiatric problems, he said, "Read between the lines."
The comments came after Bild newspaper reported that Lubitz had been treated for at least one "serious depressive episode" so bad he had to suspend flight training for several months in 2009.
The Rheinische Post also reported that the medical notes discovered in Lubitz's apartment came from at least two doctors - suggesting he may have been searching for a favourable diagnosis.
German aviation authorities said Lubitz's medical file, tied to his pilot's licence, contained a notation that he was required to have "special regular medical examinations" but such citations can relate to a wide range of medical conditions.
Yet the prospect that mental health problems may have figured in the crash of the Germanwings plane additionally shone a spotlight on what critics call flaws in the regular medical checks required of airline pilots, who must pass as many as two exams a year. Such tests, however, are largely geared towards catching physical ailments, such as vision or heart problems. Mental health tests in fitness evaluations are often cursory, sometimes amounting to little more than a written questionnaire.
"Typically, there are no tests applied to identify psychological diseases," said Andreas Adrian, an aviation doctor who evaluates Lufthansa's and other airlines' pilots in Bremen. "Maybe you are giving someone a questionnaire to answer, but of course, you can get a good actor and he can easily hide any issues."
The debate intensified over whether mental health should be more deeply probed - an effort strongly opposed by some pilot groups and others who say such a policy could add to the pressures of an already high-stress job.
More rigorous mental health testing could "uncover thousands of people who are going through difficult times in their lives and prevent them flying when they are perfectly capable of carrying out their normal day jobs", said Philip Baum, editor of the magazine Aviation Security International.
"You will have to employ far more pilots, and it would be an additional stress and could make things worse."
The possibility that Lubitz may have hidden his condition may help explain how he passed his flight training programme.
Lufthansa chief executive Carsten Spohr said his company, which owns Germanwings, was never informed of the reason for Lubitz's medical leave in 2009, a period in which Bild said Lubitz was suffering from clinical depression.
Yet, even if he did hide an illness, the fact that Lubitz passed muster at Lufthansa's demanding flight school raised additional questions. The course is meant to weed out potentially troubled men and women, using role-play scenarios in cockpits to measure reactions to conflict and stress, as well as highly personal lines of questioning to assess psychological balance. "They have to expect questions about their personal histories," said Michael Mueller, chief executive of ATTC, a company that helps prepare pilot candidates for entering flight schools, including Lufthansa's.
"How did you grow up? Did your parents divorce? How did you feel when they did?" Under existing aviation laws, any diagnosis of depression or other serious mental illness should have made it difficult for Lubitz to continue flying in Europe, and certainly not without extensive treatment. Even then, certain limitations are placed on pilots who are taking psychotropic medications - such as popular anti-depressants - including a stipulation that they not be alone in the cockpit.
