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But the chances of such an attack happening anytime soon are remote, most of the terrorism experts consulted by TIME agree. For starters, it takes a lot more money to build, research or steal a weapon of mass destruction than to hijack a plane or unleash a truck bomb. It also takes a lot more brainpower. Says Amy Smithson, a chemical and biological weapons expert at the Henry Stimson Center in Washington: "I can sit here and dream up thousands of nightmare scenarios, but there are a lot of technical and logistical hurdles that stand between us and those scenarios."
The experts also agree, however, that they must rethink their assumptions. The Sept. 11 attacks took patient planning and training; no terrorist group had ever carried out so complex a mission. "I was not at all alarmist about this threat based on the historical record," says Jonathan Tucker of the Monterey Institute of International Studies in Washington, "but given what happened, we need to reassess the threat."
Of the three major types of weapons of mass destruction, biological agents may pose the greatest potential threat, followed by nuclear bombs and chemical weapons. Here's how our experts gauge the relative dangers:
Chemical Weapons
Ranging in sophistication from rat poison to powerful nerve toxins, chemical weapons are by far the most popular among terrorists. That's because the raw materials are relatively easy to get, and the finished products don't have to be kept alive. But chemical weapons aren't well suited for inflicting widespread damage. Unlike germs, chemical agents can't reproduce, observes Tucker. "You have to generate a lethal concentration in the air, which means you need very large quantities." To kill a sizable number of people with sarin, for example, which can be absorbed through the skin as a liquid or inhaled as a vapor, you would need something like a crop-dusting plane--which is why investigators last week were so alarmed to find a manual for operating crop-dusting equipment while searching suspected terrorist hideouts. Still, to attack a city with sarin, you would probably have to fly thousands of pounds back and forth over heavily populated areas--not something easily done, especially now.
Indeed, the most devastating nonmilitary chemical attack ever, by the Aum Shinrikyo cult in Tokyo in 1995, killed only a dozen people. One reason is that the delivery method was crude: cultists dropped plastic bags of sarin (smuggled in lunch boxes and soft-drink containers) on a subway platform and pierced them with umbrella tips. Also the amounts were relatively small. Says Smithson: "Any bozo can make a chemical agent in a beaker, but producing tons and tons is difficult." Aum Shinrikyo tried to make the stuff in bulk, recruiting scientists and spending at least $10 million, but it failed.
Terrorists could try to tap into the more ample supplies of chemical arms believed to be stockpiled by Iraq and other outlaw states. But Tucker points out that the leaders of such countries would probably be reluctant to let weapons banned by international treaty out of their direct control; if they were traced back it could lead to swift retaliation. "We know Saddam Hussein is ruthless," he says, "but generally he is not reckless."
Nuclear Weapons
More than 25 years ago, in an eerie foreshadowing of the World Trade Center attack, the writer John McPhee explored with nuclear physicist Ted Taylor the question of how you could topple the Twin Towers with a small atomic bomb. Positioned correctly, McPhee reported, a nuke a tenth as powerful as Hiroshima's could knock a tower into the Hudson River.
But that assumes you could manufacture the bomb and put it into position. A terrorist would first have to get hold of some sort of fissionable material--ideally, says Princeton University nuclear proliferation expert Frank von Hippel, enriched uranium. North Korea, Iraq and Libya are believed to have uranium stockpiles but would probably be loath to let them go. A more likely source is the former Soviet Union, where bombmaking supplies are plentiful, the economy is in upheaval, and security has collapsed.
Bin Laden reportedly tried to obtain uranium from the breakaway Soviet states, but his sources bilked him, offering instead low-grade reactor fuel and radioactive garbage. Even if he had been successful, says von Hippel, it would take at least 150 lbs. of uranium plus hundreds of pounds of casing and machinery to make a weapon. "Nobody's going to be carrying a bomb around in a suitcase," he says.
Far likelier is an attack on a nuclear power plant with conventional explosives--a fact recognized by the Nuclear Regulatory Commission, which has staged mock commando raids on U.S. plants for years. Alarmingly, these war-game assaults have often succeeded, sometimes "releasing" more radiation than Chernobyl (an accident, it's worth remembering, that by some estimates caused 30,000 deaths).
Biological Weapons
Germ warfare has been around since at least the Middle Ages, when armies besieging a city would catapult corpses infected with the black plague over the walls. Today the bugs authorities most fear are anthrax (a bacterium) and smallpox (a virus). Both are highly lethal: the former kills nearly 90% of its victims, the latter some 30%. Anthrax is not communicable; smallpox, on the other hand, can be transmitted with horrifying ease from one person to another. "The feelings of uncertainty, of who is infected, of who will get infected, are the main advantages of biowarfare," says Stephen Morse of the Columbia University School of Public Health.
During the cold war, both the U.S. and the Soviet Union began developing anthrax as a biological weapon. Today 17 nations are believed to have biological weapons programs, many of which involve anthrax. Officially, the only sources of smallpox are small quantities in the labs of the Centers for Disease Control in Atlanta and at Vector in Koltsovo, Russia. But experts believe that Russia, Iraq and North Korea have all experimented with the virus and that significant secret stashes remain. Even more worrisome are reports that Russia used genetic engineering to try to make anthrax and smallpox more lethal and resistant to antibiotics and vaccines. (The U.S. put a similar program on hold.)
Whatever form the next attack takes, all evidence suggests that the nation is still largely unprepared. That's beginning to change. The NRC has plans to beef up already heightened security at power plants, and public health officials are beginning to get serious about staving off biological assaults. Last year, for example, the CDC authorized a private company to cook up 40 million additional doses of smallpox vaccine to add to the U.S. stockpile--a job that will take several years. "We also need to develop new drugs and vaccines against other organisms that might be a threat," says Dr. Margaret Hamburg of the Washington-based Nuclear Threat Initiative. "And we need to do research to better understand how some of these organisms cause disease."
Why not just vaccinate every American against every possible germ-warfare agent? That would be impractical, if not impossible, and the side effects of the inoculations would pose a significant health risk. Instead, says Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota, we should strengthen the country's public health system. After Sept. 11, hospitals in New York City were asked to report any outbreaks of unusual symptoms. Health experts know that in the event of biological attack, the earlier an epidemic is detected, the easier it is to contain.
Experts in antiterrorism share their concern. At the turn of the past century, says Brian Jenkins of the Rand Corp., epidemics of diseases like yellow fever and cholera kept health workers on their toes. Now, after a decade of cutbacks, "our ability to treat large numbers of casualties has been reduced," he says. "The notion of reinvesting to create a muscular public health system is not a bad idea, even if there is no terrorism."