Andreas Lubitz, the Germanwings co-pilot who authorities say deliberately crashed a plane with 150 people on board in the French Alps Tuesday, was "rather quiet" and showed no overt signs of his intentions, people who knew him said Thursday.

Germany's top security official said the 27-year-old German citizen wasn't known to authorities, and there are "no indications of any kind of terrorist background."

During a morning news conference, French prosecutor Brice Robin did not comment on Lubitz's religious leanings or ethnic background, saying that German authorities have taken over the investigation into the co-pilot.

A German police officer walks in front of a house believed to belong to Lubitz in Montabaur, March 26, 2015. (Ralph Orlowski/Reuters)

Police in the city of Duesseldorf, where Lubitz kept a home and the intended final destination of Flight 9525, searched the property for evidence.

On Friday, German prosecutors said they found documents at the apartment indicating Lubitz was being treated for an ongoing illness that he did not disclose to his employer. Among the papers were torn-up sick notes, including one for the day of the crash. They did not elaborate on the illness.

Lubitz had been with Germanwings since September 2013 and had logged 630 flight hours. He trained at the airline's flight centre in Bremen, Germany, and at a related facility in Phoenix, Ariz., Lufthansa CEO Carsten Spohr told a media briefing this morning.

Lubitz began his training process with Lufthansa in 2008, though that process was briefly "interrupted" for several months, Spohr said. He could not elaborate on the reasons for the break.

After completing the training process, Lubitz worked as a flight attendant for 11 months with the airline as he waited for an opportunity to fly as a junior co-pilot, Spohr said.

Lubitz "not only passed all medical tests but also his flight training, all flying tests and checks," Spohr told reporters, saying the co-pilot was "100 per cent fit to fly, without any limitations."

A sign for LSC Westerwald flight club, where Lubitz had been a member, stands in front of the club's flight tower in Montabaur. (Kai Pfaffenbach/Reuters) ​Spohr said that while Lufthansa requires all pilots to undergo a medical exam each year, those tests do not necessarily include psychological elements.

"We can only speculate on motives," he said. "We have no findings at all about why the co-pilot would do this or what made him do it."

The local government in Duesseldorf says the most recent regular security check on Lubitz, conducted Jan. 27, found nothing unusual.

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It says it has also checked with authorities in Bremen and Rhineland-Palatinate state, where his hometown is, and turned up nothing from there. It says previous security checks in 2008 and 2010 also showed no issues.

Shortly before the crash, Lubitz had engaged in "very smooth, very cool" conversation with the pilot, authorities said, citing audio recordings of the cockpit.

Lubitz deliberately sent the Airbus A320 into a descent after locking the armoured cockpit door when the pilot left to use the washroom, Robin said.

He did not respond to repeated calls from air traffic control in Marseilles or from the pilot trying to re-enter the cockpit, but could be heard breathing calmly on the audio recording before the airliner smashed into a remote French mountain.

'He seemed very enthusiastic'

According to acquaintances in Germany, Lubitz "gave off a good feeling" when he recently renewed his glider pilot's licence in the town of Montabaur, where he was a member of a flight club.

"He was happy he had the job with Germanwings, and he was doing well," said Peter Ruecker, a member of the glider club who watched him learn to fly.

Ruecker said Lubitz gave no indication during his fall visit to the club that anything was wrong. "He seemed very enthusiastic" about his career. "I can't remember anything where something wasn't right."

Lubitz apparently had a girlfriend, but Ruecker didn't have many more details about his life.

Lubitz had obtained his glider pilot's licence as a teenager, and was accepted as a Lufthansa pilot trainee after finishing a tough German college preparatory school, Ruecker said.

He learned to fly at the glider club in a sleek, white ASK-21 two-seat glider, which sits in a small hangar today on the side of the facility's grass runway.​

Ruecker described Lubitz as a "rather quiet" but friendly young man.

Facebook page deleted

Club chairman Klaus Radke said he rejected Marseille prosecutors' conclusion that Lubitz put the Germanwings flight intentionally into a descent and into the French Alps when the pilot had left the cockpit.

"I don't see how anyone can draw such conclusions before the investigation is completed," he told The Associated Press.

In a statement issued before authorities revealed his role in the crash and since taken down, the club said he had joined in his youth "to fulfill his dream of flying."

At the house of Lubitz's parents, the curtains were drawn and four police cars were parked outside. Police blocked the media from the single-family, two-story home in a prosperous new subdivision on the edge of Montabaur, a town 60 kilometres northwest of Frankfurt.

Neighbour Johannes Rossmann said Lubitz appeared to be in good health and was a regular jogger. He described the pilot as calm and low-key.

"I do not believe he killed himself and claimed other people's lives," the 22-year-old Rossmann said. "I can't believe it until it is 100 per cent confirmed."

Lubitz's family could not immediately be reached, but a recently deleted Facebook page bearing Lubitz's name showed a photo of him posing near the Golden Gate Bridge in California.

The page, which was wiped from Facebook sometime in the past two days, lists him as having several aviation-themed interests, including the A320, Lufthansa, and Phoenix Goodyear Airport in Arizona.

The Facebook page also included a link to a result in the 2011 Lufthansa half-marathon in Frankfurt, where a runner with the nickname "flying—andy" ran the course in a 1 hour, 48 minutes, 51 seconds.

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