BERLIN—Andreas Lubitz, the Germanwings co-pilot who crashed an airliner into a French mountainside, was being treated for depression, a fact he apparently successfully concealed from his employer, according to a person familiar with the investigation.

Mr. Lubitz had been excused from work by his treating psychiatrist for a period that included the day of the crash, this person told The Wall Street Journal, but decided to ignore the advice and reported to work.

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His motive remained unclear, but another person familiar with the investigation said he didn’t have a terminal illness.

While Mr. Lubitz had sought to conceal his depression from Germanwings, there was no direct evidence that the fear of losing his flying classification as being fit to fly—due for renewal in July—triggered his actions, though “this would be a plausible explanation,” the first person said.