The wave of human anthrax cases spread through contaminated mail appears to be winding down, but investigators don't appear to be much closer to discovering who was behind the attacks.
To date, there have been 17 officially confirmed cases and five suspected infections, according to the Centers for Disease Control and Prevention. Those numbers have not changed for more than two weeks. The last case, a skin anthrax infection in a 38-year-old New York Post employee, was made public Nov. 2.
"If you look at the epidemic curve … there was an initial cluster, and then a second cluster of cases, and we're at the end of that," said Jim Hughes, director of the National Center for Infectious Diseases.
For now, Hughes said, federal, state and local health officials "remain on a very high level of alert" for the possibility of further biological or chemical attacks, and will remain so "until the criminals who perpetrated this [wave of anthrax-laced letters] are caught."
But investigators have voiced frustration, and the quick capture of the person or people who sent the anthrax-laced letters to Senate Majority Leader Tom Daschle and various media outlets seems unlikely.
The trail of known contaminated letters has grown cold. The FBI has asked people to come forward if they recognize the handwriting on the envelopes, or can think of a suspect based on a likely profile released last week. According to the profile, the culprit is likely a nonconfrontational male loner with a scientific background.
Few Leads; Several Uncertainties
Investigators are citing few other active leads, but several questions:
Health and law enforcement investigators wonder how Kathy T. Nguyen , 61, contracted the inhalation form of anthrax.
Nguyen, a Vietnamese immigrant who lived in New York City's Bronx borough, was the last person to die of the disease. Tests for the presence of anthrax at her work, her home and on her clothes all have turned out negative. Health officials are awaiting results on environmental samples taken along the subway line she took to work at Manhattan's Eye, Ear and Throat Hospital.
They wonder how someone was sickened with inhalational anthrax at a State Department mail facility in Sterling, Va. Although the building gets its mail from the heavily contaminated Brentwood postal facility in Washington, health experts do not believe cross-contamination from other mail can easily cause inhalation anthrax. Therefore, they wonder if another contaminated letter may lie undiscovered in heaps of backlogged mail at the State Department facility.
Likewise, police are unsure if anthrax traces found at congressional offices in the Longworth building on Capitol Hill, or other sites, may have been caused by cross-contamination, or from yet other undiscovered letters. The Longworth building was served by the Brentwood postal facility, which processed a potent letter sent to Daschle's office in the Hart Senate office building.
Four Dead Since Oct. 5
Word of the first anthrax case came on Oct. 3. Since then, four people have died of inhalation anthrax, the most severe form of the disease.
The first fatality was Robert Stevens, 63, a photo editor for a Florida tabloid who became ill after opening a contaminated letter. Stevens died on Oct. 5. His death was followed by the deaths on Oct. 21 and 22 of two U.S. Postal Service workers who had spent time at the Brentwood facility in Washington, and Nguyen's death in New York on Oct. 31.
Six other people — in Florida, New Jersey and Washington — have been diagnosed with inhalation anthrax but are recovering.
Several other people in New York and New Jersey are recovering from the less severe skin form of anthrax, which is easier to catch. Some are believed to have contracted the disease before Stevens' case became public, but their illnesses came to light later.
In addition, a number of buildings around the country — including several postal facilities and government or media mailrooms — have tested positive for anthrax contamination or traces of anthrax (see list of "hot spots," below).
About 32,000 people have been treated with antibiotics as a precaution for possible exposure to anthrax, and thousands have stayed on the antibiotics for the full 60-day treatment cycle for those believed to have had a high risk of exposure, CDC officials said.
Investigators say the anthrax samples discovered in letters mailed to Daschle, NBC News and The New York Post are indistinguishable from one another. But that may not mean necessarily that all the anthrax in the outbreaks is identical.
"These strains in the various regions of the country that we're dealing with are indistinguishable on the basis of their antibiotic susceptibility as well as their typing using more sophisticated molecular tools," said Julie Gerberding, acting deputy director of the National Center for Infectious Diseases.
"They have some characteristics in common with several of the naturally acquired strains of anthrax that have been seen in animals in the United States and in the United Kingdom and elsewhere," she added. "But that does not provide any information about where they were derived or where someone obtained them for the particular attack."
However, investigators say the Daschle, NBC and Post letters and their handwriting also appear similar. The FBI believes the sender of all three letters tends to hold grudges — perhaps against the recipients of the mail — and is familiar with the Trenton, N.J., area, where the letters were postmarked.
"It is highly probable, bordering on certainty, that all three letters were authored by the same person," the FBI wrote as it outlined the suspect's likely traits. "In the past, the public has helped the FBI solve high-profile investigations that involved writings by coming forward to identify the author, either by how he wrote or by what he wrote. We are asking for the public's help here again in the same way."
ABCNEWS' Kendra Gahagan in Washington contributed to this report.