As the FBI hunted the source of the letter, 12 senators' offices in the Hart Senate Office Building, including Daschle's, were shut for tests expected to last two or three days. The closed offices share a ventilation system.

The Hart building, a block from the Capitol, houses the offices of 50 senators, or half the Senate. Lawmakers sent many aides home, while others found makeshift space elsewhere.

Hundreds of people who work in that eight-story building, including some senators, lined up for nasal swab tests that can detect if they have been exposed to spores of the potentially fatal bacteria. They were given three-day prescriptions of Cipro, a powerful antibiotic that can normally defeat the disease.

"It's so crazy right now, it's stupid not to get it done," said Doris Reid, who works for the building's custodial staff.

Even so, the House and Senate held regularly scheduled sessions Tuesday and there were no reports of any committee meetings being canceled or postponed.

"I think under the circumstances, the Senate is functioning quite well," Daschle, D-S.D., told reporters. "We'll be back in business in all respects within the next several days."

Senate Minority Leader Trent Lott, R-Miss., said there was a growing sentiment for the Senate to finish its work quickly and adjourn for the year. But he acknowledged that the crush of work on spending, the economy, anti-terrorism and other subjects would make it hard to finish before Thanksgiving.

Daschle said investigators have found high concentrations of anthrax in the letter, which was postmarked Oct.8 and opened by a female aide on Monday morning. Daschle was in his Capitol office, but was given a test anyway because of possible exposure to spores on aides' clothes. The test was negative, he said.

Daschle said that so far, preliminary tests have shown that none of his aides were exposed to anthrax.

But he also said investigators found that the strain on his letter "clearly was produced by someone who knew what he or she was doing."

FBI Director Robert Mueller said that to date, authorities have found no direct link to organized terrorism but have not ruled it out.

The FBI released photos of the envelope sent to Daschle and another sent to NBC's Tom Brokaw in New York, where one anthrax case has been confirmed. Both were written in similar block letters and bear postmarks from Trenton, N.J. People who attended a briefing for senators said both letters also contained threatening messages.

Health and Human Services Secretary Tommy Thompson told senators he would station Deputy Surgeon General Ken Moritsugu on Capitol Hill indefinitely to help coordinate bioterrorism policy and have ready answers for lawmakers' questions.