Top executives of Lufthansa and Germanwings airlines visited the site of last week's plane crash that killed 150 people. Speaking with reporters, Lufthansa CEO Carsten Spohr did not respond to questions about the co-pilot's medical history.

Spohr said that while his airline is learning more about the crash, "it will take a long time for all of us to understand" how the tragedy occurred.

From Berlin, Esme Nicholson filed this report for our Newscast desk:

"Spohr refused to answer questions on how much his airline had known about the mental health of Andreas Lubitz, the co-pilot suspected to have deliberately caused the crash."On Monday, Lufthansa announced that they had found an email from 2009 informing the Flight Training Pilot School about a 'previous episode of severe depression.'"These revelations are calling into question whether the aviation industry's screening process for pilots is thorough enough."

Lufthansa has said that it gave documents about Lubitz's condition to prosecutors who are investigating the crash of Germanwings Flight 4U 9525.

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