The U.S. Secret Service examined the thinking and behavior of 83 persons known to have attacked or come close to attacking prominent public officials or public figures in the United States between 1949 and 1996. Available criminal justice, court, social service, mental health, and public records were analyzed to look at how individuals got the idea of assassination and what motivated them, target selection, attack planning, communications including threats before attacks, role of mental illness in assassination behavior, and key life experiences that affected assassination behavior. While no assassin profile emerged, common behaviors and activities that potential attackers engaged in before their attacks were observed. Assassination was the end result of a discernible and understandable process of thinking and behavior. The U.S. Secret Service has identified elements of a threat assessment program based on several important steps, such as defining the problem and determining what is needed to carry out protective intelligence functions. Key functions of the protective intelligence program are to identify persons who are appropriate subjects of investigation, to assess subjects who may pose threats, and to manage subjects who are determined to potentially pose threats. Guidelines to facilitate collecting information, building a database, and sharing information are detailed. Generic functions and approaches of case investigators and threat assessment investigation procedures are described that focus on interviews, corroborated information and evidence, attack-related behaviors, and target selection. Finally, guidance is offered on how to evaluate a threat assessment and on how to manage a protective intelligence case.