Compared with most types of criminal violence, terrorism poses special data-collection challenges. To begin, the term “terrorism” yields varying definitions that are often loaded with political and emotional implications. Even different branches of the U.S. government have adopted unique definitions of terrorism (cf. Federal Bureau of Investigation, 1997; U.S. Department of State, 2001). Although government departments in some countries collect official data on terrorism (e.g., the U.S. National Counterterrorism Center [NCTC]), data collected by governments are suspicious either because they are influenced by political
considerations or because many fear that they might be so influenced. Moreover, although vast amounts of detailed official data on common crimes are routinely produced by the various branches of the criminal justice system in most countries, this rarely is the case for terrorism. For example, most offenders suspected of terrorism against the United States are not legally processed for terrorism but rather for other related offenses, such as weapons violations and
money laundering (Smith, Damphousse, Jackson, and Sellers, 2002). In addition, much primary data are collected by intelligence agents (e.g., informants and communications intercepts) and are not available to researchers working in an unclassified environment.
LaFree, Gary, and Sue Ming Yang, Martha Crenshaw. 2008. "Trajectories of Terrorism: An Analysis of Major Groups that have Targeted the United States, 1970 top 2004." Criminology and Public Policy (January): 445-473. http://onlinelibrary.wiley.com/doi/10.1111/j.1745-9133.2009.00570.x/abst...