Abstract This is an independent advisory paper prepared during the week following the September 11, 2001, attacks on the World Trade Center and the Pentagon. It was submitted to the Behavioral Analysis Program of the National Security Division, Federal Bureau of Investigation (FBI) on September 21, 2001. The paper was subsequently circulated to other intelligence agencies, both in the United States and abroad. The purpose of the advisory paper was to marshal, as quickly as possible, psychological knowledge concerning “the violent true believer” and his homicidal-suicidal state of mind. It concludes with behavioral and psychological characteristics that, when combined with factual evidence, could be used to guide the investigation of one or several individuals suspected of terrorist activities. It was not prepared as a scholarly submission; therefore, no references are cited.