Security measures were taken by aviation designers after 911 to ensure a safe cockpit door in the A320,resulting in a door designed to stop intruders and protect the pilot within.
Since the Sept. 11 attacks, flight crews no longer have keys to open the cockpit door. The door remains locked during flight. When entry is requested on the outside keypad, a buzzer goes off. The pilot can toggle the switch and unlock the door.
The CVR now seems to indicate that copilot Andreas Lubitz locked out the Pilot In Command, Captain Patrick Sondenheimer. There is audio record of him attempting to get through the invincible door.