As the FBI struggles to crack the anthrax cases - even appealing to the public for help - authorities have definitely not ruled out the possibility that the attacks may be the work of a domestic extremist group or individual.
The popular perception points to Al Qaeda or some international organization. But now, with seemingly no definitive leads in the case, law-enforcement officials are reexamining all angles.
And although no domestic terrorists have ever used anthrax in an attack, many are said to be obsessed with biological weapons - as evidenced by the hundreds of anthrax hoaxes perpetrated by right-wing groups, most often against abortion clinics.
It's quite possible, experts say, that a domestic group seized on the events of Sept. 11 as an invitation to launch their own attack.
Some even suggest that domestic extremists could be working with an international terror organization. An Al Qaeda-Aryan Nations coalition, for example, may sound unlikely, but it's not impossible, say experts, given some of their shared beliefs.
At the very least, domestic terrorists may have gained encouragement from Osama bin Laden's attacks on US interests - just as Al Qaeda members may have learned from the bombing of the Murrah Building in Oklahoma City.
"Some of our home-grown extremist fanatics have very similar ideologies to the Al Qaeda fanaticism," says Jason Pate, a terrorism expert at the Monterey Institute in California. "They both hate Israel. They both think the US government is illegitimate and heavily Jewish-influenced." Even without an official partnership, he says, "if they can feed off each other, then the Sept. 11 events could be a catalyst for a wave of Oklahoma City-like events."
Certain right-wing groups have published how-to manuals for making anthrax and other bioweapons. And several have demonstrated just how easy it can be to acquire deadly agents.
Members of the Minnesota Patriots Council, an antigovernment group, were caught in 1995 plotting to kill law-enforcement officials with ricin, a toxin they manufactured using a kit ordered from a magazine.
The Rajneeshee cult acquired salmonella from a medical-supply company in 1984 and sprinkled it on doorknobs and in supermarkets and salad bars in Oregon, sickening 751 people.
In 1995, Larry Wayne Harris, a member of the Identity Christian Church, managed to order three vials of Yersina pestis, which causes bubonic plague, from a laboratory in Maryland. Mr. Harris was caught and arrested, but three years later police found eight bags of what he boasted was weapons-grade anthrax in his car. The substance turned out to be anthrax vaccine.
Harris's ability to acquire even a nonlethal form of anthrax - and the press his actions garnered - may have inspired other right-wing groups. Since 1998, the number of anthrax hoaxes sent to abortion clinics spiked sharply.
"Larry Harris did a lot to encourage interest in the white supremacist community in anthrax," says Jessica Stern, a terrorism expert at Harvard University's Kennedy School of Government in Cambridge, Mass.
Several experts agree that the letters sent in the anthrax attacks seem to suggest a domestic perpetrator. Mr. Pate says the threatening messages are "typical of the kind of anthrax threats that have originated in this country over the last three years," adding that references to Allah seem like an obvious "smokescreen."
And what investigators have learned so far from tests done on the anthrax itself also suggests a possible domestic source. The anthrax found in all the letters is a strain common to the US, and the sample sent to Sen. Tom Daschle was treated with silica, the substance the US used in its germ-warfare program.
On the other hand, the sophistication of the Daschle sample strikes some experts as beyond the skill of most domestic groups. "There has been no evidence that anyone in the white supremacist community would be capable of preparing such a professional powder," says Ms. Stern.
Likewise, the money needed to buy the equipment needed to create such a finely milled powder would present an obstacle for many far-right groups. Although a fair amount of money is raised, "most of that cash flow has gone to organizations that are very well known to the authorities," says Michael Barkun, a terrorist expert at Syracuse University in New York. "It seems most unlikely that such groups would take this kind of risk, because they are already on the radar of law-enforcement agencies."
One way around these hurdles might be through a loose partnership with an international organization, such as Al Qaeda. Although there's little evidence to link domestic extremists and Islamic terror groups, there have been a few reports that raise the possibility. For example, both neo-Nazis and Islamic extremists, united in opposition to Israel, attended a meeting held earlier this year in Beirut, Lebanon.
And in "Others Unknown," Timothy McVeigh's lawyers speculate about a possible link between Mr. McVeigh's accomplice Terry Nichols and Ramzi Yousef, the mastermind behind the 1993 World Trade Center bombing, who also has been tied to Mr. bin Laden. Mr. Nichols married a woman from the Philippines and visited that country, a hotbed for Islamic fundamentalists, six times before 1995. According to McVeigh's lawyers, one witness reported a meeting between an American whose description matched Nichols and Mr. Yousef. Bin Laden also was said to be in the Philippines around that time.
While there's no record of any joint activity between domestic and Islamic terrorists, it's "a possibility," says Professor Barkun. Not only do they share a hatred for Israel and the US government, but they may now be drawn together through "the notion that the enemy of my enemy is my friend - or at least my ally, or a possible ally," he says.