The one thing that binds these diverse groups together is suspicion--in particular, an intense suspicion of the federal government. "They see a government plot to destroy liberty and impose tyranny," said Chip Berlet, a senior analyst with the Political Research Associates think tank outside Boston.
"From their point of view, they're going up against this sinful, evil bureaucracy . . . so destroying it is an act of patriotism," Berlet said.
In that framework, he points out, committing an act of terrorism would seem laudable, a sensible "preemptive strike" to shake up the government before it could impose a repressive New World Order.
But while many in these movements may talk approvingly of striking out at the government, few are prepared to take action. As Berlet put it: "A considerable amount of it is just bluster."
Indeed, spokesmen for the militia movement have repeatedly and emphatically insisted they have no interest in terrorism. "How can we recruit our fellow Americans if we're out there maiming and killing them?" asked John Trochmann of the Militia of Montana. "We do not advocate any kind of violence. We advocate putting our nation back together."
The problem, experts say, is that the fierce anti-government rhetoric underpinning such groups may inspire more radical members to take matters into their own hands--no matter how often their leaders publicly renounce violence.
Chuck Fenwick, a former military medic who teaches survivalist techniques, points to a recent newsletter he received from one militia group announcing that its members had been approached by someone "who advocates that 'patriots need biological [weapons] capability' which he can help them obtain."
The newsletter advises members to turn down such offers by saying, "I have no intention of doing that. I don't think you should, either." But that rather tepid rejection by the militia leadership might not deter a member from translating his group's philosophy into violence.
"I consider this a major threat," Fenwick said.
Even more of a threat are the loners who don't belong to any group--but who absorb the extremist movement's rhetoric over the Internet, at survivalist expos, on shortwave radio and through videos. Authorities cite Oklahoma City bomber Timothy McVeigh and convicted Unabomber Theodore Kaczynski as examples of loners with deadly ideologies.
"You don't need a mass movement," said Rabbi Abraham Cooper, who tracks extremist movements at the Simon Weisenthal Center in Los Angeles. "You just need a couple of people who are tuned in and turned on by this stuff. Reading it all gives them a sense of empowerment and community."
Those are just the people the FBI is most concerned about, because, by definition, they're much harder to track. They also may hold the most hard-core views, and may be most intent on making a splash with a new kind of weapon. After all, to put it crudely, 20 deaths from anthrax would be a lot more horrifying than 20 deaths from yet another pipe bomb explosion.
"You usually think of terrorists wanting the big bang of fire, smoke and visible damage," Huxsoll said. "But once they see the reaction to a biological event, terrorists will be turned on by it. . . . We're going to have copycats all over the place."
Information about how to obtain and grow biological toxins is increasingly available online.
Some potential weapons, such as bubonic plague bacteria, can be obtained only if someone is able to gain access to scientific labs--or if he is able to persuade a mail-order microbe business to ship him a sample by posing as a reputable scientist, as Harris did.
Other toxins, however, are open to abuse by do-it-yourselfers. There are manuals explaining how to crush castor beans to extract ricin. And from biology books, would-be terrorists can learn where to look for anthrax and how to grow it by nourishing it with animal blood. They don't need high-tech equipment or even college degrees to wreak havoc.
"It doesn't take a microbiologist to culture bugs, and you don't necessarily have to be a chemist to put together a toxin," the FBI's Blitzer said.
Since 1984, when officials of the Rajneesh sect in Oregon pleaded guilty to poisoning 750 residents by sprinkling salmonella bacteria in salad bars, "we keep getting small eruptions of chemical and biological" terrorist plots, Blitzer said.
He emphasized that not all are connected with far-right extremists; the salmonella incident, for example, aimed to sway local elections, and other plots seem to be devised for revenge against a personal enemy.
While scientists agree that biological toxins are easy to produce, they caution that it's harder to figure out an effective way to deliver them as weapons of mass destruction.
Anthrax, for instance, is easy to cultivate in liquid form. But dumping the liquid on a street corner probably will not kill anyone. Terrorists must figure out a way to spray the anthrax in fine particles that victims will inhale.