"This memorandum is in response to a request for information about attacks by Al Qaeda both before and after the September 11, 2001 attack on the US. The request asks for the number of attacks attributed to Al Qaeda in the two-and-a-half years before September 11, 2001, and in the two and a half years afterwards, as well as the total number of attacks by Al Qaeda before September 11, 2001. While an analysis of the number of attacks worldwide committed by Al Qaeda might seem a simple, straightforward exercise, it is not, for the reasons mentioned below. We have endeavored to provide an assessment of the evidence available and will describe our findings in the body of the memorandum."
CRS Memorandum, March 31, 2004
Federation of American Scientists, http://www.fas.org