Health officials ordered anthrax testing at thousands more mail rooms across the Washington area but continued to insist that postal customers had very little risk of falling victim to the spreading bioterror. Fresh evidence of the bacteria turned up on Capitol Hill and at a growing list of government mail rooms.

Concern over the mail delivery system mounted with anthrax now spotted at more than 10 postal facilities in the area, and health officials hoped to stop its spread with an escalation of testing and treatment.

"We're getting in front of the fire," said Dr. Patrick Meehan of the Centers for Disease Control and Prevention. But he warned that thousands more people who work with the mail will need to begin taking preventive antibiotics. "It could be an astronomical number," he said.

Three weeks into the anthrax-by-letter scare, officials said they would begin testing at between 2,000 and 4,000 sites that receive mail from the city's Brentwood postal processing facility — large apartment buildings possibly among them — and recommend antibiotics for all who work at them.

Trace amounts of anthrax were detected in the offices of three House members in the Longworth Office Building, shuttered for more than a week. Mail delivery on Capitol Hill has been suspended even longer.

The majestic Supreme Court building also fell victim to a spreading bioterrorism scare, as officials ordered it shuttered for testing after a trace amount of anthrax was detected in an offsite mail center. Anthrax was also reported at a CIA mail facility, at Walter Reed Army Medical Center and a Washington post office — all of which receive mail from the Brentwood facility.

Officials emphasized that people who work full-time with mail are at a higher risk than postal customers, who only touch the letters meant for them. For typical Americans, Meehan said, the mail poses "an incredibly remote risk." Still, they recommended that people look at their mail closely and wash their hands after touching anything suspicious.

Because Brentwood serves so much of Washington, CDC director Dr. Jeffrey Koplan was asked if residents in entire ZIP codes should potentially be put on protective antibiotics: "No," he replied. "Just plain no."

Postal workers in New York threatened a lawsuit to force the government to close a large processing center where four machines were found to be contaminated. The Postal Service plans to seal off the affected machines for decontamination at the New York facility, but say the rest of the building is safe.

President Bush said the spread of anthrax was part of a larger conflict and vowed victory. "This is two-front war, and we're going to win on both fronts," he said.

In Washington, where anthrax hit five postal workers over the last week, neither the CDC's Meehan nor other officials offered many details about the latest public health directive. They did not offer a precise estimate of the number of mail handlers covered or a list of mail facilities that would be added to the environmental testing.

Thus far, anthrax has killed three people, sickened 10 more and disrupted the lives of millions.

After three weeks of pointing to potential foreign sources for the dangerous substance, officials emphasized that the bacteria in a letter to Senate Majority Leader Tom Daschle, D-S.D., could have been made in America, as well as overseas.

At least one batch of spores could easily have been produced in the United States, by a Ph.D.-level microbiologist in a small, well-equipped lab, said White House spokesman Ari Fleischer.

"That does not rule out that it could come from a foreign location," he said, "but it certainly does expand it beyond state sponsorship or foreign locations."

Fleischer's comments about the anthrax found in a letter to Daschle marked a new emphasis, although he declined to say what clues had been uncovered to warrant the shift.

Koplan 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 Daschle letter to be responsible for anthrax in mail-handling facilities in Washington, Maryland and Virginia, he said.

The government was already testing for contamination at 200 postal service facilities along the Eastern Seaboard. Bush pledged to "move quickly to treat and protect workers where positive exposures are found."

In Washington, authorities said one local post office had tested positive for anthrax. Results from 21 other facilities are negative, Postal Service executive Deborah Willhite said, and results weren't available for 14 others.

Anthrax-tainted letters were sent to NBC News anchor Tom Brokaw and the New York Post as well as the mailing to Daschle that triggered the shutdown of office buildings across Capitol Hill last week. All three were postmarked Trenton, N.J.

Supreme Court spokeswoman Kathy Arberg announced the court was closing, following the discovery of anthrax on a filter removed from a remote mail facility.

She said the justices were among those tested, and said the court would convene on Monday in a federal appeals courtroom if their building wasn't cleared for reopening. It would be the first time that happened since the building was completed in 1935.