The DHS used The Strategic National Risk Assessment (SNRA) in support of PPD-8 to drive development of the National Preparedness Goal/System/Report. Its purpose is to help identify the types of incidents that pose the greatest threat to the Nation’s homeland security.  It is important to note that classified information has been omitted from this version of the SNAR so it is more than likely different than the one actually being used. The SNRA grouped the threats and hazards into three categories: Natural, technological/accidental, and adversarial/human-caused. I had trouble understanding if this translated into the entire group being more of a risk than another. If this is the case the highest risk to the US is an outbreak of Animal Disease and the lowest is a radiological terror attack. I find it interesting that aircraft as a weapon is listed as the top threat in the category of adversarial/human caused hazards. Historically terrorist seek to exploit an unknown weakness. Prior to the attack of 9/11 the primary threat to aircraft was hijacking and bombings. The attack exposed several weaknesses in airline security which have now been addressed increasing the chances an attack will be thwarted. I believe that terrorist will continue their attempts for this style of attack but they will strive to exploit another unknown or other unaddressed weakness for their next large scale attack. One threat that I highlighted last week from the 2012 Worldwide Threat Assessment of the US Intelligence Community was that of Cyber Attacks; which has been moved to the front of the report from almost last in 2011.  The SNRA differentiates between cyber attacks that target data and cyber attacks targeting physical infrastructure and lists both threats toward the bottom of the threats. 
This question was asked on Jan 11, 2013 and answered on Jan 11, 2013.
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