One recurring question that has been at the forefront of most intelligence agencies since the Sept. 11, 2001, attacks by al-Qaida on the World Trade Center in New York and the Pentagon just 1 mile outside Washington concerns the ongoing efforts by terrorist groups to acquire weapons of mass destruction: chemical, biological and mostly nuclear.

Each of the NBC (nuclear, biological, chemical) weapons comes with a certain advantage and disadvantage -- for the terrorist, that is.

Of the three sorts, biological weapons are quite possibly the easiest to safely reproduce in a lab, assuming one knows what to do. A biological agent, as a weapon of mass destruction or as a terror weapon, is the least expensive as well as the easiest to disseminate. A bio-agent does not need a delivery mechanism and can be transported by a single person. It can pass through customs and border guards undetected, given that it is odorless and colorless.

All that is needed to spread an epidemic of botulism, for example, or mad cow disease, is to hang around a truck stop for a few hours until a semi pulling a load of cattle on its way to market in a nearby town drives in. Wait until the driver leaves his load unattended, then scrub a previously infected rag around the railings and the mouths of a few of the cattle, and let nature do the rest. The disadvantage, for the terrorist, is that the person carrying the rag is most likely to become contaminated himself (or herself). But with no shortages of jihadists queuing up to become "martyrs," finding two or three volunteers willing to die a horrible, slow and excruciatingly painful death should be no problem.

From a financial and cost-effective perspective, biological agents remain most likely the cheapest and, in all probability, the most likely agent of mass destruction to become available to terror groups.

In their haste to leave training camps and bases of operation in Afghanistan in the wake of rapidly advancing U.S. forces, al-Qaida agents left behind piles of documents, including videotapes showing tests and effects of chemical agents on animals.

Chemical weapons are more cumbersome to produce; they require larger amounts to cause enough damage to leave a psychological scar; and they require a delivery mechanism, such as an artillery shell.

Realistically, a bio-agent can cause far more deaths than a nuclear weapon, because it is not limited geographically, unlike a nuclear bomb. For example, an infected truck driver in Omaha infects a U.S. Army sergeant he met in a diner outside Tulsa, Okla. The GI travels by plane to New York, where he changes planes and boards one bound for Frankfurt. Again he changes planes, this time flying to Kuwait, where he joins up with several members of his unit heading into Iraq. Along the way the GI will have infected scores of people at every airport between Omaha and Baghdad. Those people in turn would have traveled on to Australia, South America, Canada, every European city and other parts of the world. Within a few days people from Sydney to Seattle could start dying.

A nuclear device, on the other hand, would completely devastate the immediate area and, depending on its size, would contaminate everything in a radius of several miles, but the damage would be confined to the immediate area of detonation, plus the fallout zone; in addition, depending on the wind direction and speed, radioactive particles could be carried hundreds, if not thousands, of miles. But psychologically the image of a nuclear blast carries greater impact.

Brian Michael Jenkins, who has just released a book titled "Will Terrorists Go Nuclear?" writes, "There is no doubt that the idea of nuclear weapons may appeal to terrorists." However, Jenkins stresses: "Nuclear terror can also have another insidious effect, one that imperils our very democracy. Terrorism does pose a terrible danger, but our fear of real and imagined threats must not persuade us to diminish our freedoms or our core values. There is no tradeoff between security and liberty. One does not exist without the other."

As Jenkins points out, it is important to differentiate between real and existing threats. A perfect illustration is his description of al-Qaida: "Al-Qaida may have succeeded in becoming the world's first terrorist nuclear power without possessing a single nuclear weapon."

(Claude Salhani is editor of the Middle East Times.)