With the discovery of a fourth letter containing anthrax, this time addressed to Sen. Patrick Leahy (D) of Vermont, investigators are increasingly leaning toward the theory that a lone domestic terrorist may be responsible - someone with a grudge against the media and Democratic politicians.
Although the FBI still has not ruled out the possibility that Al Qaeda may be involved, experts say the small number of the anthrax attacks so far seems to indicate a less coordinated operation. Now, the targeting of Senator Leahy, who chairs the Judiciary Committee but is not an especially high-profile figure, may point to an attacker with strong domestic political views in line with certain right-wing extremists.
But while investigators may feel increasingly confident about the type of suspect they're looking for, they seem no closer to actually catching him. Indeed, experts say lone terrorists can often be the hardest to find, since there are no communications or financial transactions to track. Forensic tests on the latest letter could provide some new clues - a fingerprint or DNA - although tests on the previous letters have come up with nothing. But beyond this and the text of the letters themselves, all of which have been short and written in clumsy block letters, investigators may find themselves relying largely on tips from the public.
"It's certainly a very difficult investigation," says Robert Blitzer, former head of the FBI's domestic counterterrorism section. "Until this person strikes again, or unless, as in the Ted Kaczynski [Unabomber] case, someone comes forward and says, 'Hey, I think I know this guy,' it's going to be very difficult."
The Leahy letter, which investigators found late Friday while sorting through some 280 barrels of quarantined congressional mail, was mailed the same day as the letter sent to Sen. Tom Daschle, so it does not represent a new round of attacks. It had the same fictitious return address as the Daschle letter, and the handwriting was also the same.
Even before the letter was discovered, the FBI had released an official profile of the suspect, portraying him as a loner with a scientific background, access to laboratory equipment, and some knowledge of the New Jersey region where all the letters have been mailed. The FBI asked members of the public to study the profile carefully and call in with any information - an appeal it renewed over the weekend, along with offering a $1.25 million reward.
Leahy also appealed to the public for help, comparing the investigation to "looking for a needle in a haystack."
Lone terrorists - those who are unconnected, or only loosely affiliated with known extremist groups - have been responsible for some of the worst recent terror attacks. Some, like Mr. Kaczynski, have been caught as a result of tips from the public. Others, like Eric Rudolph, the suspect in a number of abortion-clinic bombings, are still at large.
The rise of these lone attackers can be linked to a right-wing terrorist philosophy that sprang up in the early 1990s, known as "leaderless resistance." Advocated in a tract by former Ku Klux Klan leader Louis Beam, it calls for "phantom cells" or individuals to simply "act when they feel the time is right," rather than taking orders from an organization.
"There has been a move toward more leaderless cells," says James Corcoran, a domestic-terrorism expert at Simmons College in Boston. These small cells or individuals "operate independently from each other, and basically just take their marching orders from what they read in the newspaper."
After Sept. 11, postings on right-wing Internet chat rooms cheered the World Trade Center attacks, Mr. Corcoran says.
Catching this kind of terrorist poses a real challenge for investigators, he adds, "because there's no one to talk, other than that lone individual."
In the case of the anthrax letters, the FBI might be able to narrow its list of potential suspects based on the attacker's scientific expertise. The anthrax sent to Senator Daschle had been processed and finely milled, indicating that whoever made it has a strong knowledge of microbiology, and possibly of germ warfare.
"This is different from Kaczynski," says Mr. Blitzer. "Kaczynski was a bomber, and he was very meticulous in terms of how he put the different devices together. But this is a whole different level of science."
The FBI has been making inquiries at labs in the New Jersey area but so far that line of investigation has failed to turn up any promising leads. That means investigators' best hope, for now, may lie with the public.
"Somebody knows this person," says Blitzer. "Just like Ted Kaczynski - when his brother saw that writing, he knew it was him. I say someone knows this guy, too, and I'd keep those written letters in the press as much as possible for people to look at."