The US anthrax mail attacks of October 2001 directed against prominent media and political figures represented a watershed terrorist event. While there were a number of anthrax, plague, and various biological agent hoaxes prior to 2001, the anthrax letters represented the first time that actual, virulent anthrax spores were used to target victims. The investigation into who sent the anthrax letters remains open, but there are some facts that should be stressed. First, it is not clear that the terrorist meant to kill people since the letters were sealed and warned that the powder was anthrax. Second, the anthrax spores had been processed to a fine powder, but were not treated with any chemicals (silica, for example) to make them disperse easier. The sophistication implicit in the spore preparations points to specialist(s) with expertise that may have come from a state-run biological weapons program or from the academic research community.
The anthrax attacks underscored the fact that public health preparedness is an integral part of national security and that preparation for bioterrorism requires a robust public health system and appropriate surveillance and countermeasures. The attacks also precipitated a reassessment of what had been a low-priority national security issue and contributed to the establishment of various security laws (e.g., USA Patriot Act of 2001, Public Health Security and Bioterrorism Preparedness and Response Act of 2002) to regulate select agents and corresponding initiatives by the scientific community to apply institutional self-governance to dual-use research. This is particularly important in light of the unprecedented increase in research into select agents since the anthrax attacks.
More recent accounts by top al-Qaeda officials and in documents discovered in Afghanistan detailed that the terrorist network has had a standing interest in biological (because of their reported "cheapness"), as well as chemical and radiological weapons. It should be stressed that actual evidence of weapons production has not been made public by intelligence sources.