Anthrax has been discovered in a third person who worked at the headquarters of a tabloid newspaper company here, federal authorities reported tonight. Prosecutors formally opened a criminal investigation into a case that has left a photo editor dead, but they have no evidence linking the anthrax outbreak to the Sept. 11 terrorist hijackings.

The third person to test positive is a 35-year-old woman who worked in the building housing the National Enquirer and five other tabloids. A swab test turned up the bacteria in her nasal passages, but she has not developed symptoms of the disease. As in the case of another infected worker, doctors expect antibiotics to eliminate the infection.

More than 1,000 newspaper employees, contract workers and their relatives have been tested for exposure to the disease. Health officials announced tonight that roughly 700 nasal swab tests have come back negative. All people given the test were offered antibiotics. Photo editor Robert Stevens, 63, whose symptoms signaled the first case, died on Friday. Friends and co-workers remembered him warmly at a memorial service today, speaking of his laughter and his generous heart.

At a late evening news conference, FBI special agent Hector M. Pesquera said investigators in protective suits will be sweeping the three-story American Media Inc. building again in an attempt to trace the movements of the third person infected, a woman who asked to remain unidentified. He said agents have found nothing connecting the anthrax contamination to any terrorist organization, but are ruling nothing out as they continue to investigate. Several of the Sept. 11 hijackers had lived in recent months in nearby communities.

U.S. Attorney Guy Lewis told reporters that investigators do not know how the AMI offices became contaminated. Promising to "bring every resource we have to bear," he said authorities have not developed "what I'd characterize as conceivable theories about how the bacteria got into the building."

Authorities said anthrax, which is not contagious, has not been found outside the AMI building, now evacuated and sealed. Lewis said the criminal inquiry aims to learn how and where the bacteria was introduced, and by whom.

Investigators working to track the particular variety of anthrax found in the three employees and on Stevens's keyboard are testing a theory that it originated in the United States, perhaps in an Iowa laboratory in the 1950s. Because the strain found in Florida has so far responded to antibiotics, U.S. authorities suspect it was not engineered as a biological weapon, but emerged from a medical research lab.

The Iowa variety, dubbed the "Ames strain," was shipped by scientists to countless laboratories across the country in past decades as a benchmark for identifying anthrax. The strain was passed around freely because it grows well in culture dishes, said Norman Cheville, dean of Iowa State's college of veterinary medicine. Researchers at the Army Medical Research Institute of Infectious Diseases at Fort Detrick in Maryland have studied it for years and have distributed it periodically to university researchers.

Indeed, the strain is so widespread in laboratories around the world that confirming it was the variety that killed Stevens would tell little about where the Boca Raton spores came from, says Ronald Atlas, dean of the University of Louisville graduate school and president-elect of the American Society for Microbiology.

As the investigation of the Florida outbreak continued today, fears about an anthrax threat reverberated around of the country. Pharmacies reported an increase in requests for ciprofloxacin (brand name Cipro), an antibiotic usually effective against the disease, and the State Department ordered all U.S. embassies around the world to store precautionary supplies of the drug for their employees.

Hospitals and emergency rescue switchboards across the nation have received telephone calls from worried people complaining of the flu-like symptoms that often accompany anthrax in its early stages. Fire-rescue teams have rushed to examine powder and packages that callers found suspicious, but have discovered no anthrax.

Part of the State Department was evacuated this afternoon after a woman in the mailroom opened an envelope that contained an unknown powder. She sounded an alarm when some of the powder fell on her shoes, said Washington fire department spokesman Alan Etter. A city hazardous materials team and members of the FBI terrorism task force determined that the substance was not hazardous.

"We're walking a fine line between prudence and panic here," Etter said.

Working to calm jangled nerves, federal and state health authorities in Florida have expressed confidence that the bacteria responsible for the infection of Stevens, mailroom worker Ernesto Blanco and the 35-year-old woman, who has not been named, were confined to the three-story building where they worked. Tests of Stevens's home and garden, as well as of his favorite bicycle routes and fishing spots, revealed no anthrax spores.

Similarly, no anthrax cases, or suspected anthrax cases, have been identified elsewhere in the country.

As the AMI employees wait for results, some have warned that conventional tests for anthrax can easily give a false positive signal in the presence of closely related bacteria belonging to the same family as the anthrax germ. It was not clear as of last night whether the gold-standard test, a genetic analysis known as PCR (polymerase chain reaction), had been applied in the Florida cases.

The Centers for Disease Control and Prevention is advising people not to hoard antibiotics or take them without cause, and not to lay in supplies of gas masks. The government agency, which shipped antibiotics to nearby Delray Beach for the people connected to American Media Inc., controls a supply large enough to treat 2 million individuals. Drugs from the federal stockpile can be shipped quickly in case of an outbreak.

Health and Human Services Secretary Tommy G. Thompson urged the health care community to resist pressure to prescribe antibiotics such as Cipro. HHS spokesman Kevin Keane said, "People should not be hoarding medicine. If it is needed, there will be plenty."

Public health officials have notified doctors and hospitals to be alert to possible cases of anthrax. The disease typically responds to antibiotics taken before major symptoms develop, leaving doctors confident that any widespread attack could be thwarted.

Several Washington area pharmacies, however, have reported increased Cipro sales since Monday, when more than 300 staff members of American Media, which publishes supermarket tabloids including the National Enquirer, the Globe and the Star, were seen on national television lining up for an anthrax test.

Responding to worried readers, American Media Inc. Chairman Chairman David Pecker said in a written statement that readers will not contract anthrax from the newspapers themselves.

The Boca Raton building, about an hour north of Miami, housed only AMI editorial operations and senior management, Pecker said. "No printing or shipping of publications was done from Boca. The printing and shipping of AMI tabloids is handled by five plants around the country, none of which are in Florida," he said.

Pharmacist Bola Adeolu, at a CVS store in downtown Bethesda, said Cipro prescriptions have climbed sharply over the last two days. People are buying the antibiotics in large quantities -- packages of 100 to 150 capsules. Others are asking what they need to do to get a prescription, he said.

"Cipro usually doesn't fly off the shelf. What we had on the shelf should have lasted us two weeks. It sold in two days," Adeolu said.

CVS spokesman Todd Andrews said the increase in Cipro sales has been most pronounced in the New York metropolitan area.

Bayer AG, the largest drug manufacturer in Germany, announced today that it will reopen a shuttered production plant to increase its output of Cipro by 25 percent after Nov. 1. The plan is a response to increased U.S. demand amid anthrax worries, said spokesperson Christian Sehnert.

At a congressional hearing today, Rep. Peter Deutsch (D-Fla.) accused Bush administration officials of being slow to release details of the investigation into the American Media case, particularly during a time of high anxiety.

"The press accounts look like something out of a bad movie," Deutsch said, waving a fistful of articles from local newspapers. "People are calling up the hazmat teams every time they see a packet of dust. . . . You're not clearing up an awful lot."

Staff writers Ceci Connolly, Terence Chea, Petula Dvorak, Sue Anne Pressley and Rick Weiss contributed to this report.

Hazardous material workers investigate a substance in a U.S. Postal Service container at a mail distribution center in Deerfield Beach, Fla., Monday, that turned out to be dirt.Fire-rescue workers bag suspected biohazardous materials at American Media Inc. offices in Boca Raton, Fla., where three victims worked.