A German co-pilot who deliberately flew a passenger airliner into a French mountainside killing 150 people was "obsessed" by the Alps and was well acquainted with the area of the crash, according to people who knew him.

Andreas Lubitz, a gliding enthusiast from the village of Montabaur, north of Frankfurt, took part in at least one class organised by his local flying club in the French Alps' province where he brought down Germanwings Flight 4U9525 earlier this week.

Ernst Müller member of the Montabaur flying club told Le Parisien that Lubitz attended a flying course they organised in Sisteron, a town in the Alpes-de-Haute-Provence, some 70km from the crash site.

Another club member told the newspaper that in such occasion the 28-year-old flew over the mountains he later chose to die on, bringing another 149 people with him.

"Andreas took part in one of these courses in the Alpes-de-Haute-Provence with my niece, who was a good friend of his," said Dieter Wagner. "He was passionate about the Alps and even obsessed. I'm sure he knew the crash area because he had glided over it".

Friends of the co-pilot's family told France's Metro News that he had been regularly visiting Sisteron and its flying club for years.

An unnamed source close to Lubitz's parents was quoted by the website as saying the pilot had first visited the area aged nine in 1996.

It was earlier reported that Lubitz picked up the passion for flying at a very young age, growing up with the dream of becoming a professional pilot.

Earlier, German authorities revealed Lubitz hid illness from his employers and was supposed to be off sick on the day of the Airbus A320 crash.

Detectives that searched Lubitz's homes in Montabaur and Duesseldorf found a torn-up sick note signing Lubitz off work on day of the French Alps crash. The "torn" illness certificate shows that Lubitz was receiving medical treatment.

They also said they could not find any suicide note or claim of responsibility in the apartment.