From what the French prosecutor was saying this morning, the co-pilot had to be intentionally preventing the pilot from re-entering the cockpit. There was a number code that the pilot could use to unlock the door (which could be used if the person still in the cockpit was unconscious), and if the person in the cockpit did nothing, the door would be unlocked--the co-pilot had to override that.
This is probably the greatest mass murder at one time by a German since the end of World War II.