The FBI on Monday took over the investigation into the anthrax death of a Florida man after the germ was found in the nose of a co-worker and on a computer keyboard in their office. Hundreds of people who worked near the men lined up to get medical tests.

Attorney General John Ashcroft said the case could become "a clear criminal investigation."

"We don't have enough information to know whether this could be related to terrorism or not," he said during a news conference in Washington.

The FBI sealed off the Boca Raton building housing several supermarket tabloids, including The Sun, where both men worked. Agents donned protective gear before going inside.

How the bacterial spores got into the newspaper's office remained under investigation. Federal investigators handling the cases have eliminated the obvious environmental sources of anthrax, said Barbara Reynolds, a spokeswoman for the Centers for Disease Control and Prevention in Atlanta.

Sen. Bob Graham, D-Fla., said CDC officials told him that "human intervention" was the likely cause of contamination.

Health officials insisted there was no public health threat, but there was unease among some of the 500 people waiting for antibiotics and anthrax tests at the Palm Beach County health agency Monday.

"I feel nervous. I'm worried for everybody," said David Hayes, an editor for the Star tabloid who works in the building. Test results are expected to take days or weeks in some cases.

Anthrax cannot be spread from person to person, but all 300 people who work in the building -- and anyone who spent more than an hour inside since Aug. 1 -- were advised to visit health officials.

Antibiotics can treat anthrax, though the form that killed Sun photography editor Bob Stevens is particularly lethal. Stevens, 63, died Friday of inhalation anthrax, the first such fatality in the United States since 1976.

The anthrax exposure case reported Monday involved a mailroom employee identified by co-workers as 73-year-old Ernesto Blanco. Health officials said he had anthrax bacteria in his nasal passages, but he has not been diagnosed with the disease.

Blanco was tested for anthrax because he happened to be in a hospital for what co-workers said was an unrelated heart problem.

He was in stable condition at a Miami-area hospital, authorities said. Relatively large anthrax spores that lodge in the upper respiratory tract are less dangerous than smaller spores that get into the lungs.

Reynolds said authorities may never know whether he actually had anthrax because antibiotics may have killed it before it was detected.

Anthrax can be contracted from farm animals or soil, but the bacterium is not normally found among the wildlife or livestock in Florida. Stevens was described as an avid outdoorsman and gardener.

"When you have two cases in the same building and a positive sample from the environment in that building and no wool sorters or animal hides in that building, it lowers the likelihood of it coming from the environment," Reynolds said, reading a statement from CDC Director Jeffrey Koplan.

State epidemiologist Dr. Steven Wiersma said tests will help determine whether the anthrax found in the second victim was natural or genetically engineered. Health officials have said the bacteria in Stevens' blood responded to antibiotics, indicating that it was natural.

He and other health officials said there was no reason for alarm.

"The risk is low," said Dr. John Agwunobi, Florida secretary of health. He said the sample of anthrax that was found in the building was taken from Stevens' computer.

The Sept. 11 terrorist attacks have raised fears of bioterrorism across the country, and focused particular concern on the origin of the anthrax here.

Stevens lived about a mile from an air strip where flight school owner Marian Smith said suspected hijacker Mohamed Atta rented planes. Several suspected hijackers also visited a crop-dusting business in Belle Glade, 40 miles from Stevens' home in Lantana.

David Pecker, chief executive of the tabloids' publisher, American Media, said he did not believe the company was being targeted by terrorists because of how the papers have covered the attacks and suspected terrorist mastermind Osama bin Laden.

Newsweek magazine reported on its Web site Monday that the office received a "weird love letter to Jennifer Lopez" a week before the Sept. 11 attacks. Inside was what was described as a "soapy, powdery substance" and a Star of David charm. The letter was handled by both Stevens and Blanco, according to unidentified workers cited by Newsweek.

Bennet Bolton, a senior reporter for The National Enquirer, told The Associated Press on Monday about a "cryptic" e-mail sent to the staff in late August or early September by an intern who worked in the newsroom this summer.

"It intrigued us that he left such a cryptic farewell," Bolton said. "It was rather neutral and then he said, 'I left you a surprise for you to remember me by. Ha ha, just kidding.'"

He said federal investigators were told about the e-mail. The FBI did not return several phone calls seeking comment about it.

Only 18 cases of inhalation anthrax were reported in the United States during the 20th century, the most recent in 1976 in California. Without treatment, 90 percent of victims die within days. More common is a less serious form of anthrax contracted through the skin.

Federal officials are sending Florida 100 cases of antibiotics to back up the local supply. The antibiotics came from a federal stockpile that holds enough to treat 2 million cases of anthrax.

An injectable anthrax vaccine has been around since the 1970s, but it limited to military use. It is reportedly not in production.