In the urgent quest to learn who is behind the different anthrax attacks of the past month - and to find the perpetrators before deadlier attacks occur - one of the most fertile sources of clues is proving to be the bacterium itself.
Already, investigators have been able to establish links between the cases.
DNA testing indicates that the anthrax in letters sent to NBC News and American Media Inc. was of the same strain - which could mean it came from a common source. While tests on the anthrax sent to Sen. Tom Daschle's office were not complete at press time, an official from the Defense Department says there is "no evidence, based on what we know thus far, that it's any different from" the anthrax in the other two cases.
That knowledge, while increasing the likelihood that the attacks are connected, does not automatically give investigators a specific place to look in determining where the terrorists got the bacteria in the first place. Many labs scattered around the world can possess samples of a single strain. Moreover, the strain found in Florida (in the American Media letter) and New York (in the NBC letter) is one that occurs in nature - meaning it could have been collected and grown by anyone anywhere.
Still, these clues give authorities a starting point in their efforts to trace the source. And just as the FBI is exploring the connection between the letters sent to NBC and Senator Daschle - both sent from Trenton, N.J., in similar envelopes - chemists and microbiologists are performing a similar type of detective work in the lab. By establishing the kind of medium the anthrax was grown in and the ways in which it was processed, experts say, they might be able to trace it back to a certain country or program.
"In this case, science is on our side," says Randall Larsen, director of the ANSER Institute for Homeland Security, a nonprofit group in Arlington, Va.
"Some people have thought that biological weapons were such a wonderful asymmetric weapon because you could do it anonymously," Mr. Larsen says, "But it's tougher to remain anonymous now. The detective story is a long way from over, and it will be interesting to see where it leads us."
Assuming investigators have a large enough sample to perform a variety of tests, "a really clever chemist might figure out" ways to identify certain markers, either on the "goop" the anthrax was mixed with or even on the bacterium itself, says Matthew Meselson, director of Harvard University's Sussex Program on Chemical and Biological Weapons Limitation in Cambridge, Mass. "This gets into real sleuthing."
There are still some puzzling discrepancies between the cases. The anthrax sent to NBC was reportedly brown and granular, whereas the substance in Daschle's office was said to be a powder. In fact, reports that the Daschle sample was unusually fine initially led many experts to speculate that it might be linked to a state weapons program, because producing finely milled anthrax requires a high degree of technical expertise.
But authorities say the sample is not resistant to antibiotics, and the Defense Department official yesterday classified it as "run of the mill."
FBI Director Robert Muller, meanwhile, announced a $1 million reward for information leading to the conviction of whomever is behind the attacks, and Postmaster General John Potter said the US Postal Service was sending a postcard to every US household detailing the kinds of things in the mail that should arouse recipients' suspicion.
Although investigators are actively exploring potential links to Al Qaeda, the network led by Osama bin Laden, they have not ruled out the possibility that a domestic group may be responsible.
Theoretically, anyone with a certain understanding of microbiology could obtain and grow a deadly form of anthrax. All they'd have to do, says Dr. Meselson, is find a place where an outbreak among livestock had occurred (just recently, there was a case in Saskatchewan, for example), and isolate the bacterium from the soil. From there, it wouldn't be all that difficult to grow the culture and purify it.
The hardest part, he says, is figuring out how to mill it down to a size that would float in the air efficiently - which the powder in Daschle's office was said to do, although the sample's degree of fineness is still uncertain.
"Only nations, probably, have figured out how to do this," says Meselson. But, he adds, this means "how to do it is in the minds of people," including former employees of weapons programs in the Soviet Union and the US, who could now be anywhere.
Certainly, it's possible that domestic terrorists are responsible for the attacks, but it seems increasingly unlikely, says Mark Pitcavage, a specialist in extremist groups who has worked for the Justice Department's antiterrorism program.
"Anthrax is harder to obtain in the US versus outside the US," he says. "In addition, the writings on the letters that I am aware of do not seem to mention any clearly domestic concerns."
If the Daschle sample is, in fact, especially fine, that would make it even less likely that the source was domestic, he adds. But that strikes him as unlikely. True weapons-grade anthrax would be "wasted on this sort of attack, when regular old anthrax would do as well."
Staff writers Francine Kiefer and Brad Knickerbocker contributed to this report.