Terrorists proved they can hit multiple American targets simultaneously with what amounts to conventional weapons: fuel-heavy airliners used as giant napalm bombs.
The questions that now worry officials and private defense specialists, after last week's attacks in New York and Washington: Could terrorists carry out other attacks, using terrible weapons of mass destruction? Could a few attackers, for instance, deliver chemical or biological agents -deadly in small amounts - to US cities, via a car or light aircraft? Could they hit one of America's 103 nuclear reactors, turning it into a pile of radioactive rubble that endangers a wide swath of the country, perhaps by flying a hijacked aircraft into it?
Officials are not warning of any imminent threat, but many experts say the United States is increasingly vulnerable to such an attack, particularly given the sophistication shown by terrorists last week.
Osama bin Laden, the suspected terrorist-mastermind and financier, is known to have sought weapons of mass destruction. And there is some evidence (dead animals seen by satellite) that his group may have tested biological weapons at training camps in Afghanistan.
The day after the destructive events of Sept. 11, the Congressional Research Service warned that "the world is increasingly moving into an era in which terrorists may gain access to nuclear, chemical, or biological weaponry." And for the first time, the National Guard dispatched nine germ-warfare units to different locations.
Less than a week before the attacks, Frank Cilluffo of the Center for Strategic and International Studies (CSIS) in Washington told the Senate Foreign Relations Committee that while "the scientific sophistication needed to sustain and deliver BW [biological warfare] agents, if not insurmountable, is substantial, nonetheless, the fabrication of a crude BW device and means of delivery ... is very realistic and difficult to detect or preempt at any time."
The fact that Mr. bin Laden may not be tied to any one country could make the current situation even more dangerous. "Unlike their state-sponsored counterparts, non-state actors are much freer from the constraints of retaliation" says Mr. Cilluffo.
Policymakers and strategists have been working to understand the complexity of the threat, as well as the current ability to respond. Earlier this summer, CSIS sponsored a bioterrorism exercise called "Dark Winter." Former Sen. Sam Nunn (D) of Georgia took the role of the president, and other present and former officials assumed high-level federal and state posts.
Among the war-game participants' findings: The government "currently lacks adequate strategies, plans, and information systems to manage a crisis of this type or magnitude."
An attack with biological weapons, they warned, could lead to "massive civilian casualties, breakdowns in essential institutions, disruption of democratic processes, civil disorder, loss of confidence in government, and reduced US strategic flexibility."
How likely is such an attack? Most experts put it in the "low probability, high risk" category. This means it could be harder to pull off than the hijacking of the four airliners, but that the consequences could be far more extreme. It also means the US needs to be all the better prepared -particularly because many of the dozen or so countries known to possess such weapons (or the means to make them) are considered to be "rogue states" that are potentially supportive of terrorist groups.
The most devastating example of chemical and biological weapons in the 20th century is Germany's use of mustard gas
during World War I. Since then, other countries (including the US) have stockpiled such weapons, many of which are deteriorating and need to be safely disposed of.
The best-known case of a recent terrorist attack involving chemical weapons was carried out by the Aum Shinrikyo cult in the Tokyo subway system in 1995. Sarin, the nerve gas released there, killed 12 commuters and made another 6,000 ill.
After that attack, the US government increased support for those on the front lines of responding -police, firefighters, public-health officials, and other emergency teams.
"US cities are making headway in preparedness," says Amy Smithson, head of the chemical and biological weapons nonproliferation project at the Henry L. Stimson Center in Washington.
But there still is much to do. As with the threat of terrorism generally, the best way to thwart a chemical, biological, or radiological attack, say experts, is to improve intelligence capabilities and nonproliferation efforts so that such attacks can be prevented in the first place.