That the co-pilot of the Germanwings Andreas Lubitz deliberately kept his colleague outside the cockpit, in order to drop the flight 4U9525 in the French Alps; A Prosecutor at heart of research says that the pilot left the cockpit was unable to re-enter. How is it possible to lock someone out from the cockpit;

The BBC explains the entry Protocol of the crew in the cockpit, and how someone can be locked except for this. The pictures are from Airbus's instructional video.