BERLIN--The Germanwings co-pilot who is thought to have crashed an airliner into a French mountainside this week, killing himself and 149 passengers and crew, was being treated for a medical condition that he appears to have kept secret from his employer Lufthansa AG, German prosecutors said on Friday.

The prosecutor said evidence collected in a search of Andreas Lubitz" Duesseldorf apartment on Thursday afternoon didn't include a suicide note and gave no indication of a political or religious motive for his apparent decision to crash the plane.

"However documents were confiscated that contained medical information indicating an existing medical treatment," the Duesseldorf prosecutor said in a statement.

Documents found in the apartment showed that Mr. Lubitz received a note from his doctor for the day of the incident excusing him from work, but that he tore up the note and didn't inform Lufthansa about his condition, according to the prosecutor's statement.

"Doctor's notes that were found that were current and for the day of the incident support the assumption, based on a preliminary evaluation, that the deceased concealed his illness from his employer and work environment," the prosecutor said in the statement.

Write to William Boston at William.Boston@wsj.com

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