Despite several overlaps between crime and terrorism, criminological examinations of terrorism to date have been limited. To fill this gap in the research, we examine several individual and contextual socio-demographic characteristics of a diverse sample of extremists operating in the United States who have committed violent crimes. In addition, we provide a comparative analysis to explain and understand differences between extremists who have committed violent crimes while active in either far-Right, far-Left (including environmental and animal rights extremists), or Al Qaeda and affiliated movements. To assess the impact of external factors on the nature of domestic extremist violence, we also comparatively examine these three types of domestic extremists before and after the September 11, 2001 terrorist attacks. We find several similarities across domestic extremists but many important suspect- and county-level differences as well. We end the paper with suggestions for future research that could extend the criminological study of terrorism.