In 2004, the President issued a homeland security directive focusing on defense against biological weapons. This directive, along with the National Strategy for Homeland Security published in 2002, mandated assessments of the biological weapons threat to the nation and assigned responsibility for those assessments to the Department of Homeland Security (DHS). To assist in this project, DHS asked the NRC to carry out a study of the methodology used by the agency to prepare its first bioterrorism risk assessment. This interim report provides a preliminary examination of that methodology along with recommendations for near-term guidance and direction for the further development of its risk analysis models. A final report will address longer-term issues in the development of risk assessment capabilities for DHS.