Federal officials suspect foul play, not an environmental source, is at the root of two Florida anthrax cases that have left one man dead and hundreds of his co-workers lining up for medical tests.

Experts virtually ruled out natural causes Tuesday after the anthrax bacterium was found in the nose of a second employee and on an office computer keyboard at American Media Inc., publisher of several supermarket tabloids.

Federal authorities are awaiting test results to determine whether the strain of anthrax that killed Bob Stevens, the 63-year-old photography editor for the Sun, was manmade or natural.

Earlier Tuesday, federal officials said they believed the bacteria was manmade. But they later said test results had not been completed by the Centers for Disease Control and Prevention.

The officials said investigators so far have found no evidence linking the Florida incident to terrorism, but they suspect it might have involved criminal activity.

A sweep of items belonging to suspected hijacker Mohammed Atta, who flew planes near the sight of the anthrax case, and a screening of all the hijackers possessions, cars and hotel rooms turned up no evidence that they came into contact with anthrax or other biochemical agents, the officials added.

The discovery of a second case followed the death on Friday of Stevens.

"The chances are one in a billion to have two anthrax cases," said Dr. Landis Crockett, director of disease control for the Florida Department of Health. "There then would be another explanation, and that would be that foul play would be suspected."

Jeffrey Koplan, director of the CDC, was also very suspicious of the Florida cases.

"I asked Dr. Koplan what would be the likelihood that such a disease could have occurred without human intervention," Florida Sen. Bob Graham said after meeting with the director. "His words were, 'Nil to none,'" Graham said.

Anthrax cannot be spread from person to person. Antibiotics can treat anthrax, although the rare inhalation form that killed Stevens is particularly lethal. Untreated, 90 percent of victims die within days. The bacterium normally has an incubation period of up to seven days, but could take up to 60 days to develop.

As the FBI sealed off American Media's Boca Raton offices, Newsweek reported on its Web site Monday that the company received a "weird love letter to Jennifer Lopez" a week before the Sept. 11 terrorist attacks. Inside the letter was a "soapy, powdery substance" and a Star of David charm.

The letter was handled by both Stevens and his exposed co-worker, 73-year-old Ernesto Blanco, according to unidentified workers cited by Newsweek. The FBI has not returned a call seeking comment on the letter.

Officials said Blanco, a mailroom employee, had anthrax bacteria in his nasal passages but has not been diagnosed with the disease. He was in stable condition, authorities said.

A third American Media employee, librarian Martha Moffett, originally tested negative for anthrax, but was called back Monday to undergo a second test for the disease, The Miami Herald reported in Tuesday's editions.

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. More than 500 people waited for hours Tuesday to be tested and receive antibiotics.

Four supermarket tabloids — the Sun, the National Enquirer, Star and Weekly World News — are published in the building.

Elsewhere, a Virginia doctor downplayed what had been described as a possible anthrax case there with connections to the Florida cases.

The Sept. 11 terrorist attacks have raised fears of bioterrorism across the country, but particular concern has been focused on this Florida neighborhood.

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

Only 18 cases of inhalation anthrax were reported in the United States during the 20th century, the most recent in 1976 in California. More common is a less serious form of anthrax contracted through the skin.

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.

The Associated Press contributed to this report.