"It is highly concentrated, it is pure and the spores are smaller," Homeland Security chief Tom Ridge said today at a White House news conference. 
Those qualities require elaborate equipment to produce and lead to a greater likelihood of the anthrax going airborne and infecting its victims with inhalation anthrax, an often-fatal disease. 
Law enforcement officials told ABCNEWS investigators are narrowing the field of who could have made the highly potent form of bacteria. 
While not ruling out a domestic source, the officials said authorities increasingly believe that someone from a state-sponsored biological warfare program provided either the anthrax itself or advice to those manufacturing it. 
That belief is based, not only on the uniformly small size of the anthrax particles and the purity of the substance, but also on the detection of an additive that keeps the particles from sticking together. 
Only a small handful of countries are known to be able to produce anthrax with those attributes: the United States, Britain, France, Iraq and the former Soviet Union. A wider group of states are suspected of having the capability, including China, Egypt, India, Israel, Iran, North and South Korea, Pakistan, Taiwan, South Africa, and Syria. 
U.S. officials say no anthrax powder has been produced in this country since the government destroyed the last stockpile in the early 1970s. 
But Ridge refused to comment on the "potential range of actors" who have the capabilities to produce the anthrax. 
The director did, however, confirm that the anthrax in the letter sent to Senate Majority leader Tom Daschle, the anthrax found in letters mailed to NBC News headquarters in New York the New York Post and that discovered at American Media Inc., a tabloid publishing company in Boca Raton, Fla., is all from a strain of bacteria known as the "Ames strain" one that was originally cultivated in Iowa, but used in research laboratories around the world.
ABCNEWS has learned that federal investigators are looking into the backgrounds of anyone who works or recently worked in research facilities that store anthrax and those that have equipment which could be used to grow and process bacteria. 
They are searching for people who have the expertise to produce anthrax-contaminated powder and who may be angry at the government or the media or sympathetic to Islamic extremists.
There are at least 66 facilities that are certified to work with anthrax in the United States, including Los Alamos National Laboratory, Fort Detrick in Maryland, Northern Arizona University and Louisiana State University. 
Officials in Washington announced a new confirmed case of inhalation anthrax today. 
An employee at a facility in Sterling, Va., which handles the State Department's mail is said to be in "guarded" condition after having been hospitalized Wednesday night. 
The source of the infection was not known, but the mail that goes to the Sterling facility is first processed at the Brentwood area facility, where an anthrax-laced letter to Daschle is known to have been processed. 
"This is the first case that we have someone who has been confirmed of a non-postal worker at Brentwood," Postal Service Vice President Deborah Willhite said at a news conference. 
A second employee at the facility at Sterling reported flu-like symptoms tonight and is being evaluated at a hospital. Officials say 80 other employees at the facility are being tested and given Cipro. A total of 250-300 employees at state facilities which receive mail from Brentwood are being given Cipro, including two inside the State Department itself. In addition mail handlers at 220 posts and embassies overseas have been told to start taking Cipro.
Two workers from the Brentwood facility died of inhalation anthrax earlier this week and two others diagnosed with the disease remain in serious, but stable condition. 
Officials have been urging mailroom workers from the roughly 200 private companies and government agencies that receive "mail in bulk" large quantities of mail bundled together for delivery from the Brentwood site to get tested for anthrax. 
That recommendation was prompted by the revelation Wednesday that a letter carrier from a postal facility in Hamilton Township, N.J. likely contracted cutaneous anthrax by handling a letter contaminated by a sorting machine. The anthrax-laced letters sent to Daschle, NBC News headquarters in New York and the New York Post were all processed at the site. 
Chief D.C. health officer Dr. Ivan Walks said the worker at the State Department facility reported that he had "never" been to the Brentwood facility. 
Test results are expected to soon determine whether a woman now being treated at Holy Cross Hospital in Silver Spring, Md., has inhalation anthrax, the most serious form of the bacterial infection. 
The patient is a journalist who works on Capitol Hill and was one of many who rushed to Daschle's office in the Hart Senate Office Building last Monday when news broke that a letter containing an anthrax-contaminated powder was opened by the senator's staff. 
Now doctors fear she may have been infected as a result. Twenty-eight people on Capitol Hill have tested positive for anthrax exposure, but this would be the first case of infection from the incident. 
In addition, in New York today, NBC reported that another employee is suspected of having a possible case of cutaneous anthrax. Health officials said she also handled the letter that was the source of the anthrax exposure at NBC and that she is on antiobiotics and back at work. 
Daschle said today that anthrax spores have been found in two new areas of the Hart building in the air conditioning duct on the ninth floor and in a stairwell leading from the eighth to the ninth floor. 
The disclosure comes a day after Capitol Police announced that traces of anthrax were detected in a first-floor freight elevator bank located in the southwest quadrant of the building.
The elevators were likely used to deliver the letter to Daschle's offices on the fifth and sixth floors, but authorities have not ruled out the possibility that a second tainted letter could be involved. 
Earlier sweeps by hazardous materials officials turned up traces of anthrax in the mailroom of the Dirksen Senate Office Building and at a mail-processing facility outside the Capitol Hill complex, where the Daschle letter likely passed through, as well as in the mailroom at the Ford building, where only House mail is known to be processed. 
Three of the half-dozen congressional office buildings across the street from the Capitol building reopened this week after being shut down last Wednesday for testing and decontamination. Officials hope to open the Dirksen building on Friday.
In addition, six truckloads of mail for the White House and Congress were shipped to Ohio today so they could be put through e-beam technology designed to kill anthrax. 
ABCNEWS has learned that some federal investigators now believe the author of the three anthrax-laced letters sent to Capitol Hill, NBC News headquarters in New York and the New York Post is a U.S. citizen, someone who has lived in the United States for a considerable length of time or someone who has studied American culture. 
Some agents point out that all three handwritten letters were dated "09-11-01," with the month preceding the day the way Americans, but not Europeans or Arabs, typically write dates. 
And the fact that one letter was sent to the Post , a largely regional newspaper, suggests the person responsible was very familiar with the Northeast and American culture. 
Even the message, "Death to America. Death to Israel. Allah is great," which all three letters contained, may be misleading. 
"I think the phrases are indicative that they are trying to divert our attention to make us look at a foreign organization, to make us look at al Qaeda (Osama bin Laden's terrorist network)," said ABCNEWS counterterrorism expert Vince Cannistraro, "but that they are trying to obscure the real origin, which appears to be domestic."