Investigators are retracing the steps of a New York woman who died from inhaled anthrax by using a number of methods, including subway computer records.

Kathy Nguyen last week became the fourth person to die from inhaled anthrax in the outbreak that began with the September mailing of tainted letters to news media in Florida and New York, and to Senate Majority Leader Thomas Daschle.

Officials feared she was the first victim of an anthrax attack by some means other than mail, but "every day that goes by without seeing another unexplained inhalational case makes it less and less likely" that happened, said Dr. Anthony Fauci of the National Institutes of Health.

Fauci said the worst of the anthrax-by-mail episode may be over. "For this episode, we're out of the woods," he said.

But another attack — either newly mailed anthrax-tainted letters or by some other means — can't be ruled out. Particularly until Nguyen's death is solved, "vigilance is heightened around the country," said Dr. James Hughes of the Centers for Disease Control and Prevention.

The tainted letters are presumed to have left a trail of contamination in post offices and government buildings along the East Coast — but traces have been found in the Midwest and as far away as Russia, in a diplomatic mailbag sent to a U.S. consulate.

Officials in New York were using a number of methods to try to track Nguyen's last moments.

"We're using several items, such as her Metrocard," said Deputy Police Chief Joseph Reznick. "We know what days she was at work; we know what hours she worked; we've seen phone records, of course. But absent those items, there's very little for us to go on."

A Metrocard is a computer-coded prepaid ticket for use in the city's bus and subway system. The system records the time and place a ticketholder used the system. Nguyen used the subway to commute from her home in the Bronx to her workplace in the stockroom at Manhattan Eye, Ear and Throat Hospital.

In Washington, thousands of pounds of mail addressed to government agencies have been piling up since Daschle's letter was opened Oct. 15. The Postal Service said Tuesday it had begun sanitizing this mail and would start delivering it within 24 to 48 hours. Two facilities, in Bridgeport, N.J., and Lima, Ohio, are cleansing about 750,000 pieces of mail a day by irradiating it.

But getting back to normal on Capitol Hill will take at least several more weeks.

Still facing cleanup is the Hart Senate Office Building where Daschle's office is located. Officials on Tuesday abandoned plans to pump chlorine dioxide gas into the entire building amid fears it may damage computers and artwork without killing all the bacteria. The latest plans are to try gassing just the offices where most of the potent anthrax spread, and do more traditional cleanup in the rest of the building, which won't reopen before Nov. 21.

Regulators are furiously trying to stem fraud arising from the public panic over the anthrax crisis.

The Securities and Exchange Commission suspended stock trading of a company that claimed it is developing an anthrax disinfectant, warning investors Tuesday to be wary of unproved bioterrorism claims. And the Federal Trade Commission disclosed it is investigating hundreds of Web sites that sell potentially illegal antibiotics, fake anthrax and smallpox remedies and fake at-home anthrax tests.

In addition to the four deaths, 13 people are fighting either the inhaled or milder skin form of the disease. But it has been eight days since the last confirmed diagnosis, and the CDC said Tuesday there are no new suspicious cases under investigation.

That doesn't mean there won't be another case or two of anthrax, cautioned Fauci, director of the NIH's National Institute of Allergy and Infectious Disease.

Tens of thousands of Americans are taking antibiotics for 60 days to prevent anthrax infection because they may have been exposed to the bacteria, which can incubate for up to two months before causing illness. But there's no guarantee that when all those people finish their drugs, a couple cases of anthrax that the antibiotics didn't cure won't appear, he said.

"Do I think it will happen in significant numbers? No. But if a case or two cropped up, I wouldn't be surprised," Fauci said.

Thus, all of those people must be alert to possible anthrax symptoms in the days after they quit the antibiotics, he said — something their doctors should have warned them about and that the CDC is gearing up to monitor.

The Associated Press contributed to this report.