1. U.S. Homeland Security Council 2007 http://www.dhs.gov/xlibrary/assets/nat_strat_homelandsecurity_2007.pdf
2. U.S. Department of Homeland Security 2008 http://www.dhs.gov/xlibrary/assets/DHS_StratPlan_FINAL_spread.pdf
3. U.S. Federal Aviation Administration 2008 http://www.faa.gov/about/history/historical_perspective/