The sick leave note was found during a search of the co-pilot Andreas Lubitz's home, torn to bits among other documents, implicating mental illness and proving that he had been receiving medical treatment, news agency reported.

Prosecution sources denied that a suicide note was found in the search, or that any information was found indicating his "political or a religious backgrounds".

In a statement, the Dusseldorf prosecution said the notes were considered proof that he hid his illness from his company and his employers.

Reports said it was known that co-pilot had prematurely dropped out of his training for a few months in 2009, triggering speculations that he may have been suffering from depression at that time.

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Lubitz, who allegedly crashed the Germanwings Airbus A320 plane en route from Barcelona in Spain to Dusseldorf in Germany began his flying apprenticeship at age 14 at a local aviation institution, and joined the Lufthansa Flight Training school in Bremen in 2007.

In 2009, he interrupted his training for a few months, which he later resumed and completed before joining the low-cost subsidiary of Lufthansa, Germanwings, in 2013.

Lufthansa CEO Carsten Spohr said on Thursday that upon resumption of his training, Lubitz passed the most rigorous tests, both physical and mental.

Earlier on Friday, a spokesman of Lufthansa, the parent company of Germanwings, told a news agency the co-pilot had passed all Lufthansa tests but declined to comment on his mental condition.

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Meanwhile, the University Hospital of Duesseldorf said in a statement that Lubitz visited the hospital as a patient in February 2015 and his last visit was on March 10, 2015.

The hospital said Lubitz was receiving a diagnosis evaluation, but refused to disclose more details. It also denied previous media reports that the co-pilot was suffering from depression.

The current priority of the investigators is to find the second black box in the hope that it will further clarify the sequence of events in the cockpit. The first box, found within a few hours of the disaster, revealed that the co-pilot may have intentionally crashed the plane, keeping the captain locked out of the cockpit.

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