The recent suicide of Army scientist Bruce E. Ivins, shortly after being implicated, brought a likely end to the Anthrax scare of 2001 but, while information on this specific case remains sealed, the Global Terrorism Database at the University of Maryland is unclassified and available online to researchers. It contains more than 85,000 terror incidents since 1970. Hundreds of details associated with each incident are included to make the tool most useful to social scientists.

It shows that the 2001 anthrax attacks in the United States may be the only ones on record.

Bio-chemical terrorist attacks are very rare, according to Gary LaFree, director of the Global Terrorism Database at the University of Maryland, part of the U.S. Department of Homeland Security-funded National Consortium for the Study of Terrorism and Responses to Terrorism (START).

The U.S. anthrax attacks stand nearly alone in our database and anthrax has been used as a terror weapon very infrequently since 1970, LaFree adds. In 1993, the Japanese group Aum Shinri Kyo attempted to disperse anthrax in Tokyo and in the area around Japan 's Parliament. Both attacks failed. In fact, there have only been a handful of bio-chemical attacks of any type on record and perhaps the most prominent ones go back to Aum Shinri Kyo's sarin attacks in Tokyo in the 1990s.

Chemical weapons provide greater hurdles to terrorists, he adds. Relative to explosives, you need a lot of bio-chemical materials to inflict mass casualties and they must be handled very carefully, LaFree says. That doesn't mean bio-chemical weapons don't pose a serious potential hazard. But so far, at least, they have not been the weapon of choice.