This report later proved erroneous, though many other toxic chemicals doubtless were being released by the blaze. Several firefighters and Apex residents reported to the hospital with respiratory problems.

Investigators from U.S. Chemical Safety and Hazard Investigation Board, an independent federal agency, are now investigating whether incompatible chemicals were improperly stored too close together at the Environmental Quality Industrial Services facility, causing the fire and several explosions. With the probe still under way, it is not clear whether the Apex fire was an accident or was intentionally set. Regardless, the North Carolina incident -- like industrial fires of the past -- raises a number of questions about the vulnerability of chemical plants, storage sites and transportation vehicles in the United States, and the potential consequences if terrorists were to strike such facilities near a metropolitan area.

In this analysis, it is crucial to recognize that the true targets in such an attack would be the populations of metropolitan areas; the chemical sites or transport carriers themselves would, like the aircraft used on 9/11, be harnessed as weapons. This consideration has some bearing on the possibility -- as opposed to the mere technical viability -- of an attack in a given location. If the targeting criteria that have been ascribed to al Qaeda in the past still apply (given the disruption of the organization and reconstitution of the threat since 9/11), strikes involving chemicals would be more likely to be attempted in metropolitan areas that carry symbolism or widespread name recognition within the Muslim world. Though vulnerabilities might apply broadly to facilities around the country, the risk in places like Jersey City, N.J., Houston, Texas, or New Orleans, La., would by this token be elevated.

Chemical Plots

Many militant groups have shown interest in developing and employing chemical weapons in their attacks. These range from Aum Shinrikyo, which used chemical devices in several attacks in Japan, to domestic militants in the United States like Texan William Krar, who in April 2003 was found in possession of a completed sodium cyanide device.

Al Qaeda also figures prominently into this list. It should be recalled that the United States launched a cruise missile against Sudan's al-Shifa pharmaceutical factory in 1998, where al Qaeda was believed to be trying to manufacture chemical and biological weapons. In 2001, Ahmed Ressam -- the Algerian national who plotted to blow up Los Angeles International Airport -- testified that al Qaeda members conducted tests using cyanide and other toxins to kill dogs at the Deronta training camp in Afghanistan. Videos recovered by U.S. troops after the invasion of Afghanistan supported this testimony, and al Qaeda training manuals that have been seized included recipes for making biological toxins and chemical agents.

Terrorists and other criminals -- indeed, anyone with an interest in chemical attacks -- today have ready access, through the Internet, to information on how to make these kinds of substances. Recipes for toxins such as ricin, and instructions for creating chemical weapons such as sodium cyanide devices, can be easily downloaded.

Despite the fear that these substances engender, however, they often are quite ineffective as weapons. Clearly, Aum Shinrikyo's efforts to carry out chemical attacks were largely a flop, for all that the group spent many millions of dollars to develop its weapons program and had access to scientific facilities. Militants in Madrid killed far more people (191) with conventional explosives -- and with far less time, effort and cost -- than Aum Shinrikyo did in its Tokyo subway attack, which claimed 12 lives.

One of the most significant challenges terrorists face in chemical weapon plots is the need to manufacture and transport chemicals in quantities sufficient to yield a significant body count. As military commanders learned on the battlefields of Europe during World War I, and during the Iran-Iraq war, chemical agents are volatile and quick to vaporize, and they tend dissipate quickly. As a result, deadly concentrations can be difficult to amass in a real-world setting. Aum Shinrikyo demonstrated, through many failed attacks both before and after the successful Tokyo strike, that small vinyl bags of chemicals simply don't cut it when the goal is to yield high body counts -- even when those bags contain highly lethal chemicals like sarin and are placed in confined spaces, such as subway cars or train station restrooms.

However, chemicals can be extremely dangerous if disbursed in large quantities that can produce deadly concentrations. Several U.S. government studies have concluded that an attack involving large quantities of toxic chemicals (which can be found in water treatments, industrial solvents and fertilizers, among other common uses) could easily kill thousands of people. The death tolls envisioned eclipse those of most scenarios involving dirty bombs or improvised chemical weapons attacks against mass transit.

Modalities of Attack

Given the difficulties of making and transporting sufficiently large quantities of toxic chemicals for effective attacks, terrorists might instead look to exploit existing supplies of chemicals -- especially when they are stored or being shipped close to large cities.

Chemicals are used in many common manufacturing and municipal processes -- for example, sewage and water treatment. Therefore, facilities that require large stores of chemicals to be kept on hand abound. Chemicals are an integral part of modern, industrial societies, and the U.S. economy depends heavily on infrastructure and the ability to transport these substances rapidly and efficiently. However, knowing this, it is entirely possible that a group such as al Qaeda -- which gravitates toward spectacular attacks with crippling economic effects -- might at some point attempt an attack against a population center, using chemicals from either a storage facility or a tanker as a weapon.

If al Qaeda -- or a cell that wanted to emulate its tactics -- were to attempt such a strike (and we are not suggesting that this is the most likely form of attack on U.S. soil), history provides examples of several possible modes: armed assault coupled with vehicular bombing, a suicide pilot who crashes an aircraft into the target or an "inside job." To date, there have been no known examples of al Qaeda deploying operatives to work and attack an industrial target from the inside, but in at least one prominent case, this was an entirely feasible possibility.

It's not hard to imagine a terrorist cell adopting tactics that already have been used in places like Saudi Arabia and Yemen (where oil facilities were targeted): Operatives traveling in multiple vehicles -- some of them converted into vehicular bombs -- rammed barricades, shot at security guards with assault rifles and lobbed grenades. The failed strike at Saudi Arabia's Abqaiq facility involved trucks marked with company logos. A strike against a chemical storage site also might involve the use of explosives to damage storage tanks, or could be as simple as opening the valves or damaging pipes to release chemicals, in hopes of creating a toxic explosion or chemical cloud.

Frankly, despite post-9/11 security improvements in the United States, many chemical manufacturing and storage facilities remain vulnerable -- especially in the event of a well-orchestrated attack by heavily armed suicide operatives, or an attack using a suicide truck bomb. Some vulnerabilities were obvious even in the aftermath of the New York and Washington attacks: In 2002 and 2003, environmentalist groups penetrated several industrial sites, dropping banners from buildings as they protested against the chemicals industry. Though these actions were not orchestrated to prove a point to the Department of Homeland Security, they did lead to the implementation of tighter security measures -- and protesting activists have not succeeded in penetrating chemical storage facilities in more recent times. Nevertheless, there are very few industrial or manufacturing facilities of any kind in the United States with security levels sufficient to withstand a determined attack by a committed group of armed militants.

As has been frequently noted, al Qaeda historically has shown a fixation with plots involving aircraft, which means manufacturing or chemical storage sites are also vulnerable from the air. Crashing an aircraft (perhaps a rented or hijacked cargo jet, or a large private jet such as a Gulfstream V or Boeing business jet) into a facility would generate the kind of drama -- if not the death toll -- al Qaeda seeks.

Alternatively, an attack could be staged as an "inside job." A number of trained engineers have become prominent members of al Qaeda in the past: Ziad Jarrah, one of the 9/11 pilots, was an aerospace engineering student at the University of Applied Sciences in Hamburg, Germany; Mohammed Atta studied civil engineering there in pursuit of an urban planning degree. Khalid Sheikh Mohammed trained as a mechanical engineer, and his nephew, Abdel Basit, earned a degree in electrical engineering.

And there was Nidal Ayyad, a naturalized U.S. citizen born in Kuwait of Palestinian parents. Ayyad graduated from Rutgers University in 1991 with a degree in engineering and was hired by Allied Signal Inc. in Morristown, N.J., where he worked primarily with chemicals used in pharmaceuticals and paints. At Allied Signal, he used company letterhead to place orders for powerful chemicals, which in turn were used to manufacture the explosives for the 1993 World Trade Center bombing.

Conceivably, Ayyad could have used his position in another way: He could have helped the World Trade Center co-conspirators plan an attack against the facility where he worked. A knowledgeable insider would be aware of which chemicals were most dangerous and where they were located. Alternatively, an insider like Ayyad could have used his knowledge of and access to chemicals to create an intentional "Bhopal incident" -- emulating the 1984 chemical disaster in India, where the accidental release of methyl isocyanate in a densely populated region claimed thousands of lives. Such an act of sabotage potentially could be more catastrophic than an armed attack by outsiders.

Al Qaeda certainly has used insiders to assist attacks before: Three of the gunmen involved in the May 2004 assault against ABB Lummus Global's petrochemical facility in Yanbu, Saudi Arabia, worked at that facility. These operatives' security badges and familiarity with the layout of the site were key elements in the attack. The objective in this strike was to kill Westerners in Saudi Arabia (and the attackers did kill five) rather than to unleash a cloud of toxic chemicals, but it does stand as a cautionary tale for security professionals who guard sensitive industrial facilities.

Separate from this, shipments of chemicals also are vulnerable to exploitation. More than 800,000 shipments of hazardous materials are moved along U.S. highways, railways and pipelines each day.

Chemicals being transported by rail cars -- which move slowly along static routes and thus can be accessed easily -- present a tremendous risk. Currently, rail tanker cars are marked in ways that make it easy for first responders, and by the same token, potential terrorists, to identify their contents in the event of an accident (though proposals to use more decoy tanker cars and thicker tank wall construction could help mitigate this risk). A successful breach of just one 90-ton tanker car could result in a chemical gas cloud measuring 41.5 miles long and four miles wide. (By way of comparison, fewer than 40 tons of chemicals are believed to have been released in the Bhopal accident.)

A study conducted by the U.S. Naval Research Laboratory estimates that, in a major city, an attack involving a railcar of volatile chemicals could cause up to 100 deaths per second as the toxic cloud spread throughout the area. Because of this, some cities have moved to ban the rail shipment of chemicals through their environs, although the legality of such a ban under interstate commerce rules is still being questioned. Even if the city were to reroute the tankers, suburban areas through which the tankers passed might still be attractive targets.

Conclusion

Al Qaeda clearly has a long and documented history of interest in weapons of mass destruction, including chemical weapons. Though al Qaeda's capabilities, as a cohesive organization or strategic force, are now more questionable than they were five years ago, the scenarios outlined here remain difficult for corporate and government security professionals to discount. A chemical attack clearly might not be the most effective means of striking at the United States, but the vulnerabilities of the system do make it a viable option.

From a security standpoint, the true fear revolves around efficiency. On 9/11, al Qaeda used "outside the box" thinking to achieve tremendous results; the resources (commercial aircraft) of its enemy were commandeered and converted into guided missiles that caused large-scale destruction. Operatives did not have to design, build or steal actual missiles, which are much more tightly guarded than commercial jetliners. In business terms, the cost-to-risk ratio was highly favorable.

It is not inconceivable, therefore, that al Qaeda or other jihadists with appropriate backgrounds would seek to commandeer another industrial asset (volatile chemicals) and convert it into a weapon to be used against a recognizable target like a major metropolitan area.

Given the targeting criteria used in the past, it could be argued that emphasis would be placed on striking symbolic targets where possible -- and few cities are, in and of themselves, as symbolic as individual buildings associated with U.S. political or economic power. However, the highest-value symbolic targets in the United States (and certainly those that, like the World Trade Center towers, also would yield high body counts) might well be beyond al Qaeda's reach. Thus, there are trade-offs to be considered: The cost and risk ratios involved in a chemical strike targeting a large city would parallel those of the 9/11 aircraft plot, and the potential death tolls easily could dwarf those yielded by striking an iconic building. Finally, in this way, the operatives would -- like skillful judo practitioners -- be once again turning the United States' economic power and inertia back upon the country as a weapon.