Confronting a mystery and a major public health challenge, scientists worked Saturday to determine how anthrax was spreading through parts of Washington, including Capitol Hill, where the bacteria has been found in 11 locations.
The search for anthrax widened considerably, with thousands of businesses and apartment complexes placed on alert.
Health officials advised anyone who processes mail in those locations to begin taking precautionary antibiotics.
By Saturday night, more than 20,000 congressional employees, postal workers and private mail handlers in and near the capital had been told to take antibiotics, even without submitting to medical screening tests, just in case.
All nine justices of the U.S. Supreme Court were taking doxycycline, an anti-anthrax shield now being recommended under some circumstances as an alternative to Cipro, city health officials said.
The officials said they were switching to doxycycline -- for people possibly exposed to anthrax but not showing any symptoms of it -- because the drug has fewer side effects and is as effective as Cipro.
The justices started taking the drug after an off-site office that handles the Supreme Court's mail was found to be contaminated. Doxycycline was being given to workers there.
On Capitol Hill, a new crowd of workers lined up throughout the day to consult with doctors and pick up 60-day doses of doxycycline.
"It's a madhouse here," said Drew Kincaid, a spokesman for Rep. Mike Pence, R-Ind., who learned Friday night that his office was contaminated by traces of anthrax.
Senate Majority Leader Tom Daschle, whose office received an anthrax-laced letter earlier this month, said he could barely "express how angry I am about these attacks" and urged the nation to brace itself for another wave of bioterrorism.
"We can't know where the next threat will come from or what form it will take," Daschle said. "But we must assume there will be a next time, and we must be prepared."
Authorities worried that there might be a second anthrax-laced letter, or more, not yet discovered in Washington.
With the nation on edge over anthrax-by-mail, the post office signed a $40 million contract to buy eight electron-beam devices to sanitize letters and packages. The equipment will be used first in Washington.
Meanwhile, about 68 tons of letters and other material from Washington were being trucked to a plant in Lima, Ohio, to be decontaminated with electron beams normally used to sterilize hospital equipment.
Anthrax has killed two postal workers in Washington, one of whom was buried Saturday, and one man in South Florida. About 15 others have been infected in those areas and in New Jersey and New York.
But it is the Washington area that has been shaken by the largest and most puzzling outbreak.
"All of us are struggling to make sense of this difficult time," Washington Mayor Anthony Williams said Saturday during the funeral for postal worker Joseph Curseen Jr.
Investigators and scientists sought to explain why tiny amounts of the bacteria keep showing up in offices and buildings far from Daschle's office, where a letter contaminated by highly concentrated and deadly anthrax was opened Oct. 14.
"I would not be surprised if they find anthrax in other offices," said Rep. Rush Holt, D-N.J., who was boarding a train for home Friday night when he learned that trace amounts of anthrax had been found in his office. "No one knows how spores get around an office building. We're learning as we go."
In the most recent discovery, experts found anthrax spores in the offices of Pence, Holt and Rep. John Baldacci, D-Maine, on the sixth and seventh floors of the Longworth House Office Building, four blocks from Daschle's office in the Hart Senate Office Building.
Anthrax spores also have been found elsewhere in the Hart building, in the Ford House Office Building, in a mailroom at the Central Intelligence Agency and in remote mailrooms that serve the White House, the U.S. Supreme Court and other government operations in or near Washington.
Most experts believe that those trace amounts are the result of "cross-contamination" -- incidental contact between the letter sent to Daschle and other letters that found their way into those offices.
If cross-contamination is responsible, that could suggest that small amounts of anthrax also exist on letters delivered to the general population in Washington and elsewhere.
If it is not a factor, other fully contaminated letters might have moved through the system and, undetected, into government offices.
"We don't know if we have cross-contamination from the original Sen. Daschle letter or if there is another letter out there that we need to be concerned about," said Dan Nichols, a spokesman for the Capitol Police force. "We are investigating this very aggressively."
In any event, experts said they did not believe that the latest discoveries on Capitol Hill posed a serious threat to anyone.
"We feel quite confident that we will see no cases of anthrax out of the congressmen's offices," said Dr. Gregory Martin, an infectious-disease expert at the National Naval Medical Center.
Further complicating the issue, an employee at a remote mail facility for the State Department in Sterling, Va., contracted respiratory anthrax last week, an often lethal variant of the disease that experts do not believe can be transmitted by cross-contaminated mail.
If they are right, that probably means another tainted letter arrived at the State Department, but none has been found. Results from environmental tests at the department and its mailroom are expected later this week.
In other developments Saturday:
Another suspicious letter turned up Saturday in Florida. The letter, on its way to Rep. Mark Foley's office in Palm Beach Gardens, Fla., started seeping white powder in the local post office. The letter, which was handwritten and had no return address, was sent to an FBI lab in Miami to be tested for anthrax.
Another postal facility was closed in New Jersey, where at least four postal workers have contracted anthrax. The main post office for Princeton was shut after investigators found traces of anthrax on a mail bin.
That office is about 10 miles from a main mail processing center near Trenton that handled anthrax-tainted letters sent to Daschle, NBC News anchor Tom Brokaw and the New York Post. That center and a satellite facility in West Trenton also have been closed.
In New York City, a postal union leader advised workers at the large Morgan Processing and Distribution Center -- where some machines have been contaminated by small amounts of anthrax -- to seek transfers to other offices.
The death of a New York City postal worker earlier this month, apparently of natural causes, is being re-examined in the wake of the discovery of anthrax at the Morgan facility. The Oct. 10 death of the postal worker, Laura Jones, was ruled by the medical examiner as due to complications related to high blood pressure.