"Domestic terrorism cases differ from ordinary criminal activity in key ways. Most importantly, unlike ordinary criminals--who are often driven by self-centered motives such as profit and tend to opportunistically seek easy prey--domestic terrorists are driven by a cause or ideology. If the motives involved eventually align with the definition laid out in 18 U.S.C. §2331(5), presumably the case becomes a domestic terrorist investigation. […] Domestic terrorists are a widely divergent lot, drawing from many philosophies and worldviews to justify their illegal acts. They can be motivated to commit crimes in the name of ideas such as animal rights, environmental rights, white supremacy, anti-government beliefs, and anarchism, for example. Importantly, the expression of these worldviews--minus the commission of crimes--involves constitutionally protected activity. As such, individuals and movements openly and 'legally' espousing such beliefs distance themselves from terrorists who use the ideas to justify their own criminal actions. From the perspective of federal law enforcement, the FBI safeguards against cases focused solely on constitutionally protected activities. All FBI investigations have to be conducted for an authorized national security, criminal, or foreign intelligence collection purpose. Investigations may not solely monitor the exercise of First Amendment rights. Finally, the Department of Justice and the FBI visualize domestic terrorism in terms of threats, not named or designated groups or actors."
CRS Insight, IN10299
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html