Which agent is most likely to be used in a chemical/biological/radiological terrorist attack through the mail?

The most likely agent to be used in a chemical/biological/radiological terrorist attack through the mail is ricin. Ricin is a potent toxin derived from the seeds of the castor oil plant and is highly toxic in small doses. Its stability in the environment and relative ease of extraction make it a candidate for use in mail-based attacks, as it can be disseminated effectively in powdered form or as a liquid.

In contrast, anthrax, while it has been used in past mail attacks, is a spore-forming bacterium that requires specific conditions for spore stability and requires more substantial amounts for significant exposure. Sarin, a nerve agent, is highly toxic but typically requires more sophisticated methods of delivery and is volatile, making it less practical for mail dissemination. Botulism, caused by a toxin produced by the bacterium Clostridium botulinum, is less likely to be used in this context as it typically needs to be ingested or injected, making aerosol or mail delivery less feasible.

The characteristics of ricin, particularly its lethality at low doses and suitability for hidden delivery methods like mail, position it as a more likely choice for such an attack.