The report examines the technical ease or difficulty for terrorists to acquire, process, improvise, and disseminate certain chemical and biological agents that might cause at least 1,000 casualties (physical injuries or deaths) without the assistance of a state-sponsored program. It also attempts to determine the extent to which the U.S. government has assessed the threats and risks posed by chemical and biological terrorism in the United States to serve as a basis for planned investments. The ease or difficulty for terrorists to cause mass casualties with an improvised chemical or biological weapon or device depends on the agent selected, some of which require highly specialized knowledge for successful handling and use. Preventive measures and medical treatments are available for some, but not all, chemical and biological agents that terrorists might use. The report recommends an authoritative threat assessment of the more likely chemical and biological agents that would be used by domestic-origin terrorists working outside a state-operated laboratory infrastructure, and a national-level risk assessment to help guide and prioritize countermeasures. Notes, figure, appendixes, references