Federal authorities announced Wednesday that a third Florida resident has tested positive for exposure to anthrax and that the case has become the subject of a criminal probe.

"There is another individual that has tested positive for presence of the virus," said U.S. Attorney Guy Lewis at a news conference that included officials with the FBI, U.S. Centers for Disease Control and Prevention and the state health department.

Video: Anthrax That Killed Fla. Man Believed to Be Man-Made

Lewis said authorities are now conducting a criminal investigation into the source of the bacteria, which has killed one man, an employee of a supermarket tabloid publisher, and hospitalized his co-worker.

A third person, a 35-year-old woman, is being treated with antibiotics at a hospital. Her condition was not immediately known.

The unidentified woman is also an employee of the tabloid company, American Media, Inc., and a nasal swab taken from her tested positive, officials said.

She was one of more than 1,000 people tested by health officials who took nasal swabs and blood tests from those who recently had been inside the AMI headquarters.

"We have reviewed results of at least 700 nasal swabs," said Dr. John Agwunobi, secretary of the state Health Department. "So far ... we have found one positive anthrax culture." He said he expects the rest of the results in one to two days.

Robert Stevens, 63, died Friday of inhaled anthrax, a rare form of the disease. Ernesto Blanco, 73, has been in a Miami hospital since Monday after he was found to have anthrax spores in his nose.

Officials previously hinted that the anthrax cases could turn into a criminal investigation but never have been so assertive about the probe's direction.

Attorney General John Ashcroft said investigators are working to identify the type of anthrax and how it showed up at the AMI office. He downplayed speculation that the anthrax may have been stolen from a lab.

FBI special agent Hector Pesquera said the anthrax contamination was limited to the building owned by AMI, which publishes six tabloids. He said the woman worked in the general area of Stevens and Blanco.

Pesquera said authorities had no evidence it was caused or created by a terrorist group but he said "this is not a time for premature conclusions and inaccurate reporting."

Health officials have emphasized that there is no public health threat. Anthrax tests at Stevens' home were negative, said Dr. Jean Malecki, director of the Palm Beach Health Department.

Preliminary testing at federal labs on the anthrax that killed Stevens has found a possible match to a strain connected to an Iowa lab, a law enforcement official said Wednesday. The official, speaking on condition of anonymity, said further testing to definitively make the match could take several days.

The anthrax bacterium normally has an incubation period of up to seven days, but could take up to 60 days to develop.

FBI investigators, donning white moon suits and gas masks, spent their third day collecting evidence in the newspaper offices of the two men whose exposure to anthrax has prompted heightened fear of bioterrorism across the country.

The search has turned up no further sign of anthrax in Stevens' office since traces were discovered on his computer keyboard.

"We understand that this is a problem, and we will bring every resource that we have to bear on this problem, and I assure you we will resolve it," Lewis said.

Pharmacists in Boca Raton said their supply of the antibiotic Cipro was running dangerously thin — or in some cases out — as worried Palm Beach County residents asked their doctors to fill out prescriptions for the anthrax antibiotic.

Meanwhile, with the heightened concern about anthrax, the only U.S. manufacturer of a vaccine to fend off the disease is facing growing criticism. Bioport Corp., under contract to provide the vaccine for the military, has been unable to ship any of its product during its three years of operation. It has been stymied by its failure to meet federal drug agency standards for its renovated plant here.

And questions have arisen over Bioport's ability to produce the anthrax vaccine, and over whether the Pentagon's exclusive contract with the company has kept newer, better vaccines from being developed.

BioPort Corp. officials insist they have made the changes necessary to produce the vaccine and on Monday plan to submit new information to the U.S. Food and Drug Administration. The FDA has up to six months to review the information, and Bioport is hopeful it could ship the vaccine for use by early next year.

The concerns also spread to newsstands. American Media executives said they had received phone calls from supermarket chains and tabloid readers who were afraid they might come into contact with the bacteria while leafing through the paper.

AMI CEO David Pecker cited a statement made by the Centers for Disease Control and Prevention on Tuesday, which said "the public is at no risk of disease from handling printed paper."

A company spokesman said some customers had asked that the paper be pulled from the store for safety reasons, but individual stores were allowed to make their own decisions.

Company employees began undergoing blood testing on Wednesday at the company's Delray Beach offices, leaving with gauze bandages on their arms and small American flags in their hands.

The Palm Beach County Health Department ordered the building closed for 30 days on Wednesday, said Boca Raton Fire Department Lt. Frank Montilli.

The Associated Press contributed to this report.