From a public policy viewpoint, these uncertainties mean the United States must prepare for a wide variety of low-probability attacks on the United States, rather than to emphasize any given form of attack or group of attackers. The United States must plan its homeland defense policies and programs for a future in which there is no way to predict the weapon that will be used or the method chosen in which that weapon will be delivered. That delivery can range from a small suicide attack by an American citizen, to the covert delivery of a nuclear weapon by a foreign state. There is no reason the United States should assume that some convenient Gaussian curve or standard deviation will make small- or medium-level attacks a higher priority over time than more lethal forms.
The U.S. government is still deciding how to come to grips with these problems and how to assess possible methods of attack. A GAO report that summarized CIA and FBI views on these issues reached the following conclusions, although it must be stressed that the analysis focused on the normal historical pattern of actions by terrorists/extremists, and largely excluded attacks by state actors, proxy attacks, or covert attacks:
The possibility that terrorists may use chemical or biological materials may increase over the next decade, according to intelligence agencies. According to the Central Intelligence Agency (CIA), interest among non-state actors, including terrorists, in biological and chemical materials is real and growing and the number of potential perpetrators is increasing. The CIA also noted that many such groups have international networks and do not need to be