The still-incomplete information released about the anthrax mailed to Senate Majority Leader Tom Daschle's office in Washington, as well as to news media offices in New York and Florida, shows only that the attackers mastered the steps necessary to mount a small-scale biological assault.
But officials do not know whether the attackers have access to large quantities of anthrax spores or a delivery system capable of infecting large numbers of people.
Confusion was widespread yesterday after news media reports that the anthrax delivered to Capitol Hill was "high-grade" or "professional."
Such terms are imprecise, and experts said yesterday that it is impossible to know what they mean until detailed results of tests done on the anthrax samples are made public.
A top Army biodefense official, Maj. Gen. John Parker, said yesterday that the anthrax from Daschle's office consisted of "pure spore." He said the anthrax strain had not been fully identified but was not resistant to antibiotics.
The letters sent to Daschle and NBC News, which the Associated Press said included advice to take medication, seemed designed to sow panic, not to kill, according to several experts. Nor was the tiny quantity of anthrax that can be delivered by letter capable of mass killing.
"With a letter, you're not going to infect a lot of people," said David R. Franz, a veterinarian who spent 23 years as a top official in the U.S. Army's biological defense program at Fort Detrick. "And it's not covert. It gives you a chance to treat people."
David Siegrist, who studies biological terrorism at the Potomac Institute for Policy Studies, agreed. "It doesn't appear these guys' concern was to infect as many people as possible," he said.
"A little bit of perturbation caused by opening the envelope was enough to put the particles into the air," said Franz, who is now at the private Southern Research Institute in Frederick.
He cautioned, however, that a person with detectable particles in his nose would not necessarily have inhaled enough anthrax spores into the lungs to cause a full-blown case of the disease. In any case, the Senate employees exposed to the bacteria are taking antibiotics and should not get sick, he said.
"Weaponizing" anthrax - turning it into a form that can be used to kill - is a multistep process that begins with the acquisition of bacteria. Some natural strains of anthrax are far more virulent than others, so a terrorist might seek a particularly deadly strain.
But the seed bacteria could be obtained with relative ease - stolen from a lab, purchased from a microbiological supply house abroad, diverted from a nation's bioweapons program or collected at the site of an animal anthrax outbreak.
A sick animal's blood is loaded with bacteria, and living anthrax spores can be dug from the ground where infected animals were buried, even after a century has passed.
The bacteria might be exposed to one or more antibiotics and bred to make it resistant to treatment. That would make the anthrax more dangerous, but might also interfere with its virulence and other characteristics, said Franz, so a terrorist might not attempt it.
The next step would be to increase the volume of bacteria by growing more. To produce a relatively small amount, anthrax could be grown in a lab on a common growth medium such as blood agar, a substance made from ocean algae enriched with blood. A single plate could produce enough anthrax to put poison in many envelopes.