Additional information on aircraft security, September 11, 2001 events and actions undertaken by airworthiness authorities are available from a number of sources.
FAA Flight Standards Service, Operations Specifications MMELs Policy Letters web site at http://www.opspecs.com. The following final or draft policy letters are applicable:
PL-110 (final), SUBJECT: Supplemental Flight Deck Door Security Devices, Original dated May 24, 2002