-Chemical weapons: These are considered the most likely to be used in a terrorist attack because they are easier to make and can kill people quickly. They include nerve agents such as sarin and VX, blister agents such as mustard gas, and cyanide. Making and storing them in any quantity still require the technical capability, roughly, of a pharmaceutical company, experts say.

-Biological weapons: These cause disease. Anthrax is considered a likely candidate for an attack. Effects can be minimized with quarantines and quick medical care.

-Radiological weapons: These spread radioactive material over a wide area. Unlikely to kill anyone quickly, they will instead cause long-term problems, such as cancer, in an affected population. They could also render an area uninhabitable for a long period. They don't have the mass, rapid killing effect that terrorists seek.

-Nuclear weapons: No terrorist group is believed to be able to develop these, although there are fears that one could be obtained from Russia. Reports of suitcase-sized nuclear weapons have not been verified.