"The comprehensive increase in measures to enhance aviation security following September 11, 2001, led to significant improvements in existing security processes, operations, and technologies in each area of the aviation transportation system. These efforts led to the current security posture of a multilayered, scalable, and flexible aviation security system that is responsive to varying threat levels as well as to the entire range of identified threats. This has effectively reduced vulnerabilities within the aviation transportation system. However, the ever-changing aviation threat environment continues to challenge the Federal Government and private industry to implement additional effective and efficient security measures. As directed by the National Infrastructure Protection Plan (NIPP), the Transportation Sector- Specific Plan (TSSP) represents the combined planning contributions of the sector's security partners to develop a system wide approach to reducing the security risks within and across the transportation modes. The aviation mode's security partners include the Department of Homeland Security's (DHS) Transportation Security Administration (TSA), the Department of Transportation's (DOT) Federal Aviation Administration (FAA), airlines, the Department of Defense (DoD) airports, flight crews, air cargo industry members, State and local law enforcement, and passengers."
Dept. of Homeland Security: http://www.dhs.gov/