Lufthansa said co-pilot Andreas Lubitz, who deliberately crashed the plane in the French Alps killing 150 people on board, had informed his flight school he had a "depressive episode" in 2009.
The German airline said the note was found in emails that Lubitz sent to the Lufthansa flight school when he resumed his training after a break.
According to the Associated Press, the company had forwarded the documents on to prosecutors and declined to comment further in the case.
It comes amid reports that earlier in March, investigators in Dusseldorf found in his apartment a torn-up sick note from his doctor signing him off from work on the day of the crash. Germanwings said it never received such a sick note on the day of the doomed flight.
Lufthansa, the parent-company of Germanwings, previously said Lubitz was "100% fit to fly."
Dusseldorf's prosecutor office's said several years ago before he obtained his licence the co-pilot "was in a long period of psychotherapeutic treatment with noticeable suicidal tendencies".
There is now a Facebook campaign (titled in German) "Andreas Lubitz A320, we are against the hunt" over claims he is being framed by the authorities as a cover up for plane's mechanical failings, the Daily Mail reported.