DÜSSELDORF, Germany (AFP/REUTERS) - The co-pilot thought to have deliberately crashed his Germanwings jet into the French Alps, killing all 150 aboard, kept secret the fact that he was given sick leave on the day, German prosecutors said on Friday.

"Documents with medical contents were confiscated that point towards an existing illness and corresponding treatment by doctors," said the prosecutors' office in Duesseldorf, where the pilot lived and where the flight from Barcelona was heading.

"The fact there are sick notes saying he was unable to work, among other things, that were found torn up, which were recent and even from the day of the crime, support the assumption based on the preliminary examination that the deceased hid his illness from his employer and his professional colleagues," they said, without specifying the illness.

The prosecutors said in a statement that the documents were found in searches of Lubitz's homes in Duesseldorf and in the town of Montabaur in the state of Rhineland-Palatinate.

They said that "interviews on this subject and the evaluation of medical records will take several more days", and that the outcome would be made public "once reliable evidence is available".

The authorities did not find a "suicide note or a confession", or any evidence that the co-pilot's actions may have been motivated by "a political or religious background".

Officers had on Thursday combed through a flat Lubitz kept in Duesseldorf as well as the house where he lived with his parents in the small western town of Montabaur.

The black box voice recorder indicates that Lubitz, 27, locked his captain out of the cockpit on Tuesday and deliberately sent Flight 4U 9525 crashing into a mountainside, French officials say, in what appears to have been a case of suicide and mass murder.

French Prime Minister Manuel Valls said that "everything is pointing towards an act that we can't describe: criminal, crazy, suicidal".

Bild daily earlier reported that Lubitz sought psychiatric help for "a bout of serious depression" in 2009 and was still getting assistance from doctors, quoting documents from Germany's air transport regulator.

The paper also cited security sources as saying that Lubitz and his girlfriend were having a "serious crisis in their relationship" that left him distraught.

Lufthansa chief executive Carsten Spohr said that Lubitz had suspended his pilot training, which began in 2008, "for a certain period", before restarting and qualifying for the Airbus A320 in 2013.

According to Bild, those setbacks were linked to "depression and anxiety attacks".

Lubitz lived with his parents in his small home town of Montabaur in the Rhineland and kept an apartment in Duesseldorf, the city where his doomed plane was bound on Tuesday.

Duesseldorf prosecutors said the evidence found in the two homes "backs up the suspicion" that Lubitz "hid his illness from his employer and his colleagues".

They said they had not found a suicide note, confession or anything pointing to a "political or religious" motive but added it would take "several days" to evaluate the rest of what was collected.

Lubitz locked himself into the cockpit when the captain went out to use the toilet, then refused his colleague's increasingly desperate attempts to get him to reopen the door, French prosecutor Brice Robin said.

According to Bild, the captain even tried using an axe to break through the armoured door as the plane was sent into its fatal descent by Lubitz.

This could not be immediately confirmed, but a spokesman for Germanwings told Bild that an axe was standard emergency equipment on board the aircraft.

The tragedy has already prompted a shake-up of safety rules, with several airlines, including German companies, announcing a new policy requiring there always be two people in the cockpit.

Meanwhile, the UN world aviation body stressed that all pilots must have regular mental and physical check-ups.

German Chancellor Angela Merkel said the findings that Lubitz appeared intent on crashing the plane added an "absolutely unimaginable dimension" to the tragedy, in which most victims were German and Spanish nationals.

In the northwestern town of Haltern, which lost 16 students and two teachers who were returning from a school exchange, the revelations prompted shock and rage.

The principal of the stricken school, Mr Ulrich Wessel, said "what makes all of us so angry (is) that a suicide can lead to the deaths of 149 other people".

German President Joachim Gauck, a Protestant pastor, attended a memorial service in Haltern Friday and also extended special condolences to the families of the victims in Spain and other countries.

Meanwhile in Montabaur, Mayor Edmund Schaaf urged reporters encamped in the community to show restraint with Lubitz's parents, a banker and a church organist, who live in a handsome home on a leafy, normally quiet street.

"Regardless of whether the accusations against the co-pilot are true, we sympathise with his family and ask the media to be considerate," he said.

Investigators say Lubitz's intention was clear because he operated a button sending the plane into a plunge.

The French prosecutor played down the likelihood of Lubitz accidentally taking the plane down with an involuntary turn of the descent button.

"If you passed out and leaned over on it, it would only go a quarter-way and do nothing," Mr Robin said, adding Lubitz, who had worked for the Lufthansa group since 2013, had set the controls to "accelerate the plane's descent".

For the next eight minutes, Lubitz was apparently calm and breathing normally.

"He does not say a single word. Total silence," Mr Robin said.

The second-in-command had all psychological tests required for training, Lufthansa's Spohr told reporters Thursday, insisting: "He was 100 per cent airworthy."

Recovery operations at the crash site were ongoing, with French officials trying to find body parts and evidence. A second black box, which records flight data, has not yet been recovered.

"There's not much plane debris left. There's mainly a lot of body parts to pick up. The operation could last another two weeks," said police spokesman Xavier Vialenc.