Andreas Lubitz, the co-pilot of crashed Germanwings plane, has hid the details of a mental illness from his employers, according to German prosecutors.

The prosecutors said they found torn-up sick notes in his home, including one covering the day of the crash of the Airbus A320, but didn’t say what his illness was.

In a statement released on Friday, prosecutors said they had seized medical documents from Lubitz's apartment in Duesseldorf and the home he shared with his parents in the town of Montabaur, which “”support the preliminary assessment he concealed his sickness from his employer and colleagues”.

Investigators believe Lubitz, 28 has deliberately locked the captain out of the cockpit before sending the aircraft into a steep descent and crashing it against the side of a mountain in the French Alps some 100 km north of Nice, killing himself and the other 149 people on board on Tuesday.

Germany’s Federal Aviation Office said on Friday that a medical certificate issued to Lubitz that  allowed him to fly noted that he had a medical condition. The document, however, didn’t specify whether that condition was related to a psychological issue.

Police and rescue workers on Friday continued to comb the crash site for victims’ remains, along with other clues and DNA that could help them identify those who died in the crash.

France’s Prime Minister Manuel Valls called on Lufthansa, the parent company of Germanwings, to reveal as much information as possible about the worst aviation disaster in 15 years to help understand why Lubitz “got to the point of this horrific action.”