A New York Times article was released this evening that raised the possibility that one of the pilots was locked out of the cockpit during a terrifying 8 1/2 minute descent. This fact raises the possibility that the crash was not caused by a catastrophic malfunction (like an explosive decompression) but instead could have been an intentional crash into terrain.
How could a pilot get locked out of his or her own cockpit?
After 9/11, all airliner cockpit doors were required to be reinforced. Prior to 9/11, cockpit doors were weak and flimsy. The did not provide any protection in the event of a hijacking attempt. The upgraded doors that were installed after 9/11 could not be broken down by force. The upgraded doors also have a fairly advanced electronic security system to permit authorized access but prevent any unauthorized entry. The Airbus video below explains how the Airbus cockpit door is operated. Based on this Airbus instructional video posted on YouTube, it does appear to be possible to keep the door locked from inside the cockpit if the pilot at the controls was actively attempting to do so. While it is way too early to speculate on the exact cause, this latest information (if true) indicates that the cause of the crash could be much more sinister than mechanical.