Alps crash co-pilot Andreas Lubitz hid the details of an existing illness from his employers, German prosecutors say, BBC reported.
They said they found torn-up sick notes in his homes, including one covering the day of the crash.
In their report, Duesseldorf prosecutors did not say what illness Mr Lubitz had.
But German media have said aviation authority documents suggested he suffered depression and required ongoing assessment.
Prosecutors said there was no evidence of a political or religious motive to his actions, and no suicide note was found.
Mr Lubitz and 149 passengers and crew died when Germanwings flight 4U 9525 crashed in the French Alps on Tuesday.
Data from the plane's voice recorder suggest Mr Lubitz purposely started an eight-minute descent into mountains as the pilot was locked out of the cockpit.
In their statement, prosecutors said they seized medical documents from Mr Lubitz's two residences - his Duesseldorf flat and his parents' home north of Frankfurt - which indicated "an existing illness and appropriate medical treatment".
But "the fact that, among the documents found, there were sick notes - torn-up, current and for the day of the crash - leads to the provisional assessment that the deceased was hiding his illness from his employer", the report states.
Germanwings, a subsidiary of Lufthansa, refused to comment on the new information, the Associated Press reported.
Earlier on Friday, German media reported that Mr Lubitz's notes say he suffered a serious depressive episode when he finished training in 2009.
He went on to receive treatment for a year and a half, the German newspaper Bild reports.
Internal documents quoted by Bild and German broadcaster ARD say a note on Mr Lubitz's aviation authority file recommended regular psychological assessment.
Mr Lubitz's employers have confirmed that his training was interrupted for several months six years ago, without explaining why.
Lufthansa chief Carsten Spohr has insisted that Mr Lubitz was only able to resume training after his suitability was "re-established".
"He passed all the subsequent tests and checks with flying colours," Mr Spohr was quoted as saying.