As investigators probe why a young German pilot deliberately crashed an Airbus A320 passenger jet into the French Alps on Tuesday, pilots and psychologists warned there is no foolproof way to prevent similar incidents in the future.
All 150 people on board Germanwings Flight 9525 died after 27-year-old first officer Andreas Lubitz locked the cockpit door, took control of the plane and veered it down from cruising altitude at 3,000 feet (914 meters) per minute.
German tabloid Bild reported on Friday that Lubitz received psychiatric treatment for a "serious depressive episode" six years ago, and the crash has prompted calls for more rigorous mental health and stress tests for pilots.