In a significant shift, US authorities are again being forced to reevaluate the scope of the anthrax threat posed by the mail.
It comes in the wake of two new cases found among civilians in New York and New Jersey - the first cases of illness involving people who do not work for the Postal Service or the media.
Although the Postal Service has already warned the public to take extra care in handling mail, until now public-health officials had viewed the main risk as coming from the possibility of other letters containing anthrax, not the threat of cross-contamination.
But given the number of federal buildings in Washington that have tested positive for trace amounts of anthrax, and that investigators so far have not turned up any new letters, it now seems possible that other mail may have become contaminated with enough spores to potentially cause an infection.
While most experts believe that the low levels of anthrax found in most of these buildings does not pose a serious threat to individuals, this could mean that households whose mail passes through the same postal facilities as anthrax-laced letters could also be exposed to the bacteria.
Until yesterday, "there was no evidence at all" that an individual could be infected in their home simply by opening mail that had been processed in the same facility as one of the letters containing anthrax, Dr. Anthony Fauci of the National Institute of Health said at a press conference yesterday. Now, authorities are "intensively investigating" that possibility.
The federal buildings where anthrax has been found all receive their mail from the city's Brentwood facility, where the letter sent to Sen. Tom Daschle was processed, and where several postal workers have since been diagnosed with the disease.
The New Jersey woman who has come down with the skin form of the disease works near a mail facility in Hamilton, N.J., where at least three letters containing anthrax were handled, and where a postal worker has contracted the inhaled form of the disease.
In the case of the Brentwood facility, most experts believe that the high-speed sorting machines worked to spread the anthrax, either from the Daschle letter or another undiscovered one, into the air. Two employees from that facility have died.
"It's very clear that there was aerosol contamination from the machinery in the buildings," says Martin Hugh-Jones, an anthrax expert at Louisiana State University. But for letters passing through that machinery to pick up enough spores to sicken individuals is surprising, he says.
For an individual to contract the disease under such circumstances seems unusual. "The only thing that occurs to me is that ... they got the spore on their finger, and then scratched their forehead or their face," he says.
The amount of anthrax that has been found in federal buildings in Washington is low enough that experts still believe it would pose little risk to individuals. And so far, no employees working in those buildings have tested positive for exposure.
Thus, while it's possible that certain homes whose mail passes through the same postal facilities as those federal buildings might also test positive for trace amounts of the bacteria, that doesn't mean the levels would be high enough to harm individuals.
In addition, Deputy Postmaster General John Nolan said that the letters sent to government agencies were more likely to be susceptible to cross contamination than letters sent to ordinary citizens, because of the specific machines they came in contact with.
"We still think you ought to open your mail, and you ought to use the postal system," said Tom Ridge, director of homeland security.
More unusual is that the New York case involves inhalation anthrax. The victim, a hospital employee, spent some time in the hospital's mailroom but did not recall any suspicious letters or packages.
Normally, experts say, it takes the presence of several thousand spores for a person to develop the inhalation anthrax - way more than would likely stick to the outside of an envelope through cross-contamination. But the medical data on just how many spores are needed to cause an infection is seen by many as unreliable, based on experiments conducted decades ago, on monkeys.
Dr. Hugh Jones points out that all of the victims who have developed inhaled anthrax so far have been older, whereas those who have been diagnosed with the cutaneous version have been relatively young. This corresponds with what occurred in the Sverdlovsk anthrax outbreak in Russia in 1979, where all of the victims who developed the inhaled form of the disease were over 45, he says. It's possible that older people could be more susceptible to lower doses, he adds.
On the other hand, it's equally possible that there are more letters containing anthrax which have not yet been discovered by law-enforcement officials. "I think we've got to wait to see if there's any other cases, to see if this indicates a third wave of letters," says Hugh-Jones.