Senators filtered back to work Thursday in the face of an anthrax threat that prompted closure of the House for a sweep by teams hunting for the potentially fatal germ.

In New York, a CBS News employee was found to have been exposed to anthrax, a discovery that pointed to a broader attack on the media.

Tests performed on some 1,200 people at the Capitol complex were likely to show at least a few more were exposed to anthrax, beyond the 31 confirmed, said Sen. Bill Frist, R-Tenn., a doctor. He expressed confidence that early treatment would succeed against any who might end up infected.

Tests have found that at least 31 people in the Hart Senate Office Building across the street from the Capitol were exposed to anthrax Monday when a powdery substance fell from a letter to Senate Majority Leader Tom Daschle, D-S.D.

In New York, CBS said a woman who works for anchorman Dan Rather tested positive for anthrax. The network said the woman was exposed on the skin, not through inhalation, and is being treated. Skin exposure is less serious than inhalation.

That discovery pointed to a still broader attack on the media than had been known.

An NBC employee who works for anchorman Tom Brokaw and the 7-month-old son of an ABC News producer also have tested positive for anthrax exposure. In Florida, one man who worked for American Media Inc., a tabloid newspaper, has died from anthrax contamination and another is in hospital.

In Washington, the threat prompted a divergent course at Congress -- the Senate staying open for at least a limited session and the House closing until Tuesday.

Sen. Susan Collins, R-Maine, drove up to the Capitol on Thursday with an armload of outgoing constituency mail and some issue briefing papers.

"Most of us are only having skeletal crews today," she said. "But I think it's important that we're in session. It sends a strong signal that the terrorists or whoever is behind these attacks is not going to be successful in disrupting the government."

But House Minority Leader Richard Gephardt, D-Mo., defended the "prudent and careful" decision to shut House operations.

"We're doing what any other building anywhere in the country should do" in a similar situation, he said. "We've got to resume normal life but we've got to be careful and vigilant and we've got to make sure we're not putting people in harm's way."

In what may be the first case of tainted mail outside the United States, officials in Kenya said Thursday a letter mailed to an unidentified recipient in their country from Atlanta has tested positive for anthrax spores.

In Washington, employees on Capitol Hill lined up Thursday morning for more testing and to receive an antibiotic if necessary.

"The terrorists are in a way making some progress here," Sen. Joseph Lieberman, D-Conn., said on CBS's "The Early Show." "Have the terrorists ... created some fear? Yeah, they have. But ultimately they have created more unity in this country and on Capitol Hill than I have ever seen."

U.S. officials say they do not know whether the anthrax exposures in Washington, New York and Florida were the work of foreign or domestic terrorists.

While the only known confirmed exposures were among Senate staff, House Speaker Dennis Hastert's employees had reported suspicious mail in their suite of offices Wednesday. Gephardt said a letter sent to Hastert or his staff was being tested for anthrax.

In New York, the midtown Manhattan office of Gov. George Pataki was shut Wednesday after an initial test detected anthrax. "The odds are very high" that testing will confirm the presence of the bacterium, Pataki said.

The American Medical Association urged physicians to quit prescribing unnecessary Cipro, an antibiotic used to treat anthrax, to Americans who might be stockpiling it as a precaution.

And the Food and Drug Administration said it was about to issue specific instructions on how to use two other widely available antibiotics -- doxycycline and penicillin -- to prevent anthrax infection.

Health and Human Services Secretary Tommy Thompson asked Congress for money to stockpile 300 million doses of vaccine for smallpox, another potential bioterrorism disease.

The Senate employees known to have been exposed to anthrax so far are 23 members of Daschle's personal staff, five law officers and three aides to Sen. Russell Feingold, whose office adjoins Daschle's in the Hart building.

Daschle said none of those who tested positive for exposure was ill, although they were taking medication.

In shutting down operations for the weekend to allow for extensive testing, House leaders originally thought the Senate would join them. "To ensure safety, we thought it best to do a complete sweep, an environmental sweep," said Hastert R-Ill.

But senators decided to work on from the Capitol.

In private, angry House lawmakers and aides said Senate leaders succumbed to pressure to stay from senators who did not wish to be seen as fleeing. Senate aides called it miscommunication and said House leaders acted prematurely in deciding to leave.

At the federal Centers for Disease Control and Prevention, Dr. David Fleming said preliminary testing indicated the strain of anthrax found in a letter addressed to Brokaw in New York "appears to match the strain in Florida. Fleming said it is not yet clear whether the Washington anthrax came from the same strain.

Maj. Gen. John Parker of the Army's testing laboratory at Fort Detrick, Md., said the powder in the Daschle letter contained a "common variety" of anthrax. But Scott Lillibridge, a federal expert on bioterrorism, said there's "been some attempt to collect it, perhaps refine it and perhaps make it more concentrated."

Three government officials said Wednesday there was no evidence of any foreign involvement in the powder contained in the letter to Daschle, although they continued to investigate that possibility. One official said there was evidence that could point toward a domestic culprit.

Sen. John Kerry, D-Mass., said the letter to Daschle said: "You've been exposed to anthrax. You're going to die." He said the Daschle aide who opened the letter then dropped it, and others congregated around.