A trace amount of anthrax was discovered earlier Friday at the Central Intelligence Agency's mail-handling building at its McLean, Va., headquarters. A spokesman described the anthrax as "medically insignificant" but said the building was closed for additional tests. 
"It's not enough to cause inhalation anthrax," said spokesman Bill Harlow. He said several agency employees who handle mail are taking antibiotics as a precaution. 
Also, a test for anthrax in a mailroom in the Walter Reed Army Institute of Research in Silver Spring, Md., came back positive. The institute, which doesn't care for patients, is three miles from the hospital at the Walter Reed Army Medical Center. 
And in Virginia, a State Department mail handler who works far from previous anthrax-infected sites was hospitalized with the inhaled form of the disease. 
Three weeks into the nation's age of anthrax, the head of the Centers for Disease Control and Prevention said it was all but certain that at least one piece of tainted mail in the nation's capital remained undiscovered. 
It would be "highly unlikely to virtually impossible" for the letter to Daschle to be responsible for the known spread of the spores to mail handling facilities in Washington, Maryland and Virginia, said Dr. Jeffrey Koplan. 
Investigators are continuing their search for a sender but reporting little no progress. 
The White House said Friday that the anthrax sent to Senate Majority Leader Tom Daschle last week could have been produced by any microbiologist in a well-equipped lab. 
Press Secretary Ari Fleischer said the spores sent to Daschle were refined and highly concentrated. But he said analysis showed the material was not the most sophisticated possible. 
Fleischer said this does not rule out production by a terrorist group or foreign government, nor does it eliminate the suspicion that Osama bin Laden and his alleged terrorist network may be responsible. 
Fleischer spoke a day after Homeland Security Chief Tom Ridge reported that the strain of anthrax in the Daschle letter was the same as letters sent to New York and Florida. 
The good news, Ridge said, was that none of the bacteria had been altered genetically, meaning those exposed can be treated with antibiotics. 
The circle of infection widened with the diagnosis of the State Department employee, who worked more than 20 miles from Brentwood, Washington's central mail processing facility. Until Thursday, all those infected in the nation's capital had been tied to this central plant, which handles mail for federal agencies. 
The 59-year-old man, hospitalized in guarded condition with inhaled anthrax, worked in Sterling, Va., where about 90 percent of the State Department's mail is processed, some of which comes from Brentwood. 
Doctors asked him if his job ever took him to Brentwood. "His answer was 'never,'" reported Dr. Ivan Walks, Washington's chief health officer. 
A co-worker at the Virginia facility was declared free from anthrax after overnight tests. 
Their workplace was being tested for anthrax exposure Friday, as were 250 to 300 workers, with about 80 of them immediately given preventative antibiotics. Another half dozen State Department locations also were to be tested, including two inside the Foggy Bottom headquarters. 
The number of confirmed infections reached 13, all linked to the media or the mail, all in Florida, New York, New Jersey or metropolitan Washington. 
Among those with inhalation anthrax, three have died, three are hospitalized and one has recovered. Another six people have been diagnosed with the highly treatable skin form of the disease. 
Despite the disturbing anthrax news, officials urged calm. Many lawmakers were returning to their Capitol Hill offices, shuttered for the past week, while others were still shut out as anthrax decontamination continued. 
"We are here to conduct the nation's business. We will not be frightened," Secretary of State Colin Powell said as he appeared before a Senate committee.