The anthrax bacteria that killed a New York hospital worker match the spores used in the biological attacks on Capitol Hill and two Manhattan news outlets, according to preliminary lab tests completed yesterday. But that was the only clue made public on a day in which anthrax spores were found in four mailrooms of the Food and Drug Administration in Rockville and a post office in Kansas City, Mo.

As the anthrax crisis entered its second month, authorities expressed relief that there have been no new cases reported since Kathy T. Nguyen, 61, was diagnosed with pulmonary anthrax on Sunday. But they remained frustrated in their attempts to discover the source of the attacks.

Two days after Nguyen's death, city and federal health investigators said they had interviewed 250 of her co-workers and close contacts, and had no evidence linking her death to contaminated mail, which has been implicated in most other cases. They also said they have yet to detect the bacteria in her home or workplace.

What appeared to be a good lead initially -- anthrax spores found on Nguyen's clothing -- is proving of little help because all her possessions were mixed in a bag that tested positive. There was no way to determine if the bacteria had landed on the woman recently or had been stuck to, say, her shoes, for weeks.

"It's not that great a clue," said Julie Gerberding, acting deputy director of the National Center for Infectious Diseases at the Centers for Disease Control and Prevention. As confounding as the investigation has been, she said, officials took some consolation in the lack of new cases since the weekend.

"It's somewhat reassuring that this was not something that posed a broader threat," she said.

In Washington, where there have been no new local cases in more than a week, officials voiced similar hopes. Ivan C.A. Walks, the District's health director, suggested that the area was "on the downside" of its anthrax crisis.

The Supreme Court, which had to close its building last week and conduct its business elsewhere because tests found anthrax spores in court mail facilities, announced yesterday that the results of subsequent environmental testing in its building came back negative. The court building will remain mostly closed today. It will be open for business on Monday but closed to the public except for people attending the morning oral argument in Mickens v. Taylor.

The CDC total for confirmed anthrax cases stood at 16 yesterday, and there are six suspected cases, Gerberding said. New York City officials, using a looser standard to define "confirmed," count three more cases of the skin form of the illness, known as cutaneous anthrax.

On the law enforcement front, two men living near a postal facility in Trenton, N.J., that was contaminated with anthrax bacteria were detained on immigration violations, officials said yesterday.

Federal authorities had questioned the pair at an apartment complex in Hamilton Township, N.J., on Monday before turning them over to the Immigration and Naturalization Service. The FBI removed 10 large bags of possessions from their apartment but refused to identify the men.

The men were taken into custody at the Greenwood Village Garden Apartment complex, which was once home to Mohammad Aslam Pervez, another figure in the government's wide-ranging terror investigation. Pervez has been arrested and charged with lying to the FBI about financial transactions.

Neighbors at the complex described the two detained men as Pakistanis who have lived there since 1997, working at a nearby gas station and convenience store. This summer, said neighbors, a large number of Arabic-speaking men began congregating at their apartment.

The U.S. Postal Service announced it was offering free flu shots to all 800,000 employees nationwide in an attempt to avoid an epidemic that has symptoms similar to anthrax disease. Federal health officials fear flu sufferers with fever and aches will flood emergency rooms fearful they have contracted anthrax.

As has been the case virtually every day since the first anthrax case emerged a month ago, thousands more Americans began taking antibiotics as a preventive measure. About 1,100 individuals who worked, visited or were treated at Manhattan Eye, Ear and Throat Hospital, where Nguyen worked in a basement stock room, were offered one of the three medicines used to treat anthrax.

At a post office in Kansas City, 200 workers were given medication after anthrax spores were found on two bags of trash in the underground Stamp Fulfillment Services Center.

Authorities suspect that the bacteria may have come from bundles of mail sent from the Brentwood facility in Northeast Washington. Batches of 7,000 letters are routinely shrink-wrapped and shipped to the fulfillment center to be stamped for collectors. The working theory, said Kansas City health director Rex Archer, is that anthrax spores settled inside dust on those packages.

So far, Archer said, no one from the facility has shown symptoms of the disease. "We're feeling more and more comfortable by the minute," he said.

Anthrax spores have also been discovered on a printer sent from a post office in Trenton, N.J., to a repair company in Indianapolis, said Peter Beering, terrorism preparedness coordinator for Indianapolis. About 100 workers at DDD Co.'s Critical Parts Center have been given antibiotics. Similarly, mailroom workers at the four contaminated FDA buildings in Rockville may begin antibiotics as a precaution.

The Postal Service is testing 230 facilities nationwide; of the 47 that have been checked for anthrax, eight tested positive.

Because scientists have no idea at what levels anthrax bacteria become a health hazard, said Alan Zelicoff, senior scientist at the Center for National Security and Arms Control at Sandia National Laboratory in Albuquerque, the widespread environmental testing is, at a minimum, causing confusion.

Say, for example, environmental testing locates four anthrax spores in a room that is 10,000 square feet. "What is the hazard of those four colonies?" he said. "I'd say it's zero. We know you need several thousand to get sick." Announcing that a single anthrax spore has been found in a building "just causes panic," he said.

Health authorities also faced mounting confusion over who should be tested for anthrax poisoning and who should be put on antibiotics.

"We do not recommend nasal swabs for people who are involved in exposure when it was more than a few days in the recent past," said Gerberding. She stressed that nasal swabs will not determine whether people have the disease; they will only indicate very recent exposure to the bacteria.

"If we're not doing a nasal swab, it's not because we don't care about them, or we're not concerned about their health and safety. It's because it is not a useful test," she said.

At his daily press briefing, New York Mayor Rudolph W. Giuliani said that nasal swab tests of 28 of Nguyen's co-workers, as well as environmental testing at Manhattan Eye, Ear and Throat Hospital, returned negative.

Health officials said they planned to explain to Nguyen's neighbors in the South Bronx that they are safe from the disease. Based on the negative test results from her apartment, "we don't have any evidence that there is a public health threat," said New York City Health Department Commissioner Neal L. Cohen. "We don't have any evidence of anthrax or anthrax exposure there."

Cohen said Manhattan Eye, Ear and Throat Hospital would remain closed indefinitely.

"You have to have a greater concern about that mailroom, given that that individual was working in very, very close proximity to that mailroom," said Cohen. "Until we can rule that out as a hospital facility where people go in for treatment surgical procedures . . . we want to be absolutely sure."

Staff writers Christine Haughney, Charles Lane, Susan Levine, Robert E. Pierre, Ellen Nakashima and Susan Schmidt contributed to this report.

A member of the FBI's hazardous materials response team takes supplies from a truck outside the Bronx apartment building where Kathy Nguyen lived. Officials searched for evidence in Nguyen's apartment after she died of anthrax.Hazardous material experts leave the Hart Senate Office Building, site of an earlier letter with anthrax spores.