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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."
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.