The growing number of postal workers who have become ill from anthrax - or succumbed to it - is raising pointed questions about the true grade of the material involved, which may hold far-reaching implications about who's behind the attacks.
Though public-health officials last week sought to play down the danger posed by the substance contained in a letter to Sen. Tom Daschle (one even called it "run of the mill"), experts say the government has conspicuously failed to address the key question of particle size, which would indicate whether the anthrax is "weapons grade" or not.
Particles small enough to be easily inhaled, as those in Senator Daschle's office and the Washington-area and New Jersey postal facilities seemed to be, have only been achieved by a small number of state weapons programs.
In the absence of more definitive statements from health officials, some members of Congress - who just last week were characterized as overly alarmist in their reactions to the attack - are again referring to the anthrax as high grade. They're now pointing to a highly advanced, possibly even state-sponsored, perpetrator.
"This is weapons-grade material," said House minority leader Richard Gephardt (D) of Missouri yesterday, after a breakfast with President Bush. "It's small in diameter, which means it's been milled."
Some of the confusion to date has stemmed from various definitions of the term "weapons grade." Last week, Tom Ridge, director of homeland security, said that the anthrax found in Daschle's office had not been "weaponized," meaning it had not been treated in any particular way to keep the particles from clumping together. He also stressed that it was not resistant to antibiotics.
But experts say that "weapons grade" does not refer to the quality or virulence of the substance, but merely its ability to be aerosolized and dispersed.
"When you get a significant quantity of material coming out into the air just by opening a letter, that's indicative of material that becomes readily airborne," says Richard Spertzel, a former head of the United Nations biological inspection team in Iraq. "And if you have readily airborne material that's disseminated to that large an extent, that's weapons-grade material."
The fact that the sample sent to Daschle's office was said to billow out of the envelope - and that within 24 hours some 30 people, including two from another office, had tested positive for exposure - seems to fit with that description.
Likewise, the number of postal workers infected seems to indicate a type of material that readily disseminates. Authorities have confirmed at least two cases of inhalation anthrax and two deaths resulting from the disease among Washington-area postal workers. Yesterday, a New Jersey postal worker was also identified as possibly contracting inhalation anthrax.
This kind of material, says Dr. Spertzel, is extremely difficult to create. "That's not something that's going to be done by a home-grown terrorist," he says. "It's somebody that's connected with an existing or former weapons program, or the material was supplied to them from an existing or former weapons program."
A possible source for such material, he says, could be Iraq. While the UN inspection team never found direct evidence that Baghdad produced finely milled anthrax, they had proof that it had managed to refine a related organism to particles of less than 10 microns - a size, Spertzel says, that would stay airborne.
Another possible source is the former Soviet Union. Just yesterday, the Bush administration announced an agreement with Uzbekistan to remove deadly anthrax spores from an island where they had been secretly buried by Moscow in 1988.
Some experts suggest that the government may have withheld information about the nature of the latest anthrax outbreak - if it really did - because of the sensitive foreign-policy implications. Others say it may have not wanted to alarm the public.
Some, like Jason Pate, a bioterrorism expert at the Monterey Institute in California, say authorities may simply be "having a hard time" analyzing the material in the lab.
He notes that in the case of the letter sent to Reno, Nev., state officials and the Centers for Disease Control went back and forth for days about the test results before finally agreeing the substance wasn't anthrax.
Of course, it's still possible that the Daschle sample isn't a higher grade of anthrax than the materials found at American Media Inc. in Florida and NBC News in New York.
While the sample in Daschle's office exposed a far greater number of people, it's possible that could have more to do with how the letter was handled. "Maybe it is a better, more finely milled powder, or maybe it's just how it happened," says Mr. Pate.
In the case of the postal workers, experts say the machines used for sorting mail might have sprayed any powder that leaked from an envelope.
On the other hand, if the culprits turn out to be members of the Al Qaeda network, it's possible that different cells might be involved, using different grades of material in each letter. The variation between the attacks could even represent a deliberate escalation.
US officials still say they do not have concrete evidence linking the anthrax attacks to Al Qaeda. But yesterday, White House spokesman Ari Fleischer said that's the "operating suspicion."
Still, experts on domestic terrorism say it's not out of the question that a US extremist group might be involved. They suggest the government might even withhold certain details about the anthrax to give the attackers a false sense of security.