To turn anthrax from an animal disease into a bioterror weapon capable of killing large numbers of people is a demanding scientific task that requires some knowledge of microbiology and lab equipment, according to experts and scientific reports.
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.
Their most significant achievement was to grind anthrax spore to the size at which it quickly disperses in the air, specialists say.
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.
Dangerous particle size
But both men said that if the anthrax spores delivered to Daschle's office lodged in the nasal passages of 34 Capitol Hill employees, as tests indicated yesterday, the attackers have achieved a particle size that is very dangerous.
"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.
To make enough germs to kill thousands of people, terrorists would need a much larger, more sophisticated fermentation tank, carefully controlling temperature, nutrients, oxygen and carbon dioxide.
Such tanks were built for past U.S., British, Soviet and Iraqi weapons programs, and they were capable of producing hundreds of gallons of anthrax slurry.
"It's not like you just pour all the ingredients into a blender and turn it on," Franz said. "It's not only a science. It's an art."
Preserved by drying
Whether prepared on a small plate or in a big tank, the anthrax would then be dried by freezing it or warming it. "Anthrax is easier to dry than some things because it's so stable," Franz said. "It can take a little heat."
Exposure to air and deprivation of nutrients also converts the bacteria to the spore state - an extremely hardy form designed to survive drought and age.
"That's the way it survives. It's like a plant making seeds," says Dr. William M. Nelson, a former government scientist and now president of Tetracore Inc., a Gaithersburg company that makes anthrax test strips.
But the dried spore mixture contains large grains and clumps. So it must be "milled," or ground into particles. For a weapon designed to produce inhalation anthrax, the ideal particle size is about 1 to 5 microns. A micron is one thousandth of a millimeter; a typical human hair is 50 microns wide.
"If it's too big, your nose filters it out. If it's too small, it may just go in and go back out without lodging in the lung," Nelson said.
Anthrax is considered one of the most dangerous biological agents partly because, once the spores are dried and milled, they can be placed in a sealed container and transported from one country to another. As long as they are protected from heat and light, they will live for weeks or months.
Delivery is unpredictable
Most experts consider the final step - delivery - to be the most difficult. To kill on a large scale, the anthrax powder must be released in such a way that an aerosol cloud of a certain density settles onto crowds of people.
And to get a fatal dose, each person would have to inhale roughly 5,000 to 10,000 spores deep into his lungs.
The spores could be poured from the roof of a tall city building, sprayed from a hand-held sprayer, released in a subway or delivered from a crop-dusting plane equipped with special nozzles. But the results would be highly unpredictable and much would depend on the weather.
"Meteorology is everything," Franz said. On a windy, sunny day, the sun could kill much of the anthrax and the wind dilute it to levels that would not infect anyone. But under certain weather conditions, the particles could become an invisible, odorless aerosol, spreading at just the right density to kill on a frightening scale.
A 1993 report by the U.S. Congressional Office of Technology Assessment estimated that the release of 100 kilograms (about 220 pounds) of anthrax spores upwind of Washington could kill 130,000 to 3 million people.
That is the catastrophe scientists fear could lie at the end of the road that started this month with a few envelopes of suspicious powder.