While conventional weapons, such as explosives and firearms, remain the most likely means by which terrorists might attempt to harm U.S. civilians, the possibility of an attack involving biological or chemical weapons has increased. Many nations and terrorist groups have explored the use of such weapons on small and large scales, and many countries, including the United States, have chemical and/or biological weapons programs or materials used in these types of weapons.

Biological warfare–related technology, materials, information, and expertise—including information on potential U.S. vulnerabilities—have become more readily available. Genetic engineering is only one of several technologies that might allow countries or groups to develop agents, such as modified viruses, that would be difficult to detect and diagnose or that could defeat current procedures for protection and treatment. Furthermore, all the materials needed to produce such agents are dual-use in nature, meaning they have both military and civilian applications, so they are readily available. Any country with political will and competent scientists can produce agents. The threat from chemical warfare may also grow in coming years. Many states have chemical warfare programs, and these capabilities will likely spread to additional states and terrorist groups. Government officials consider smaller-scale bioterrorist events to be more likely than large-scale ones because they are less difficult to engineer. However, federal public health agencies, such as the Centers for Disease Control and Prevention (CDC), have little choice but to prepare for a variety of attacks.