Even when we have identified terrorist foes, we cannot always forecast and foil their attacks. Probabilistic predictions are simply not possible. The most one can do is offer assessments of comparative likelihood. Mentions of Sarin and anthrax in the press increased twenty-fold during the 1990s and, not surprisingly, we have suffered a spate of anthrax hoaxes. Hoaxes involving the threatened or alleged use of chemical or biological substances will remain our most common problem.
Attacks involving chemical agents seem more likely than biological attacks.
Attacks involving crude dispersal techniques in contained environments seem more likely than poisoning cities. Wholesale slaughter is not easy. Attacks involving ten to hundreds of fatalities are more likely than catastrophic attacks of more than a thousand deaths. But again, there are no guarantees. Uncertainty regarding the identity, intentions and capabilities of potential terrorist adversaries the traditional components of threat assessment has not reduced the sense of urgency in current programs to meet the threat.