Federal officials widened their probe yesterday into the death of a Florida man from anthrax, acknowledging that they are considering bioterrorism after anthrax spores were found on the victim's computer keyboard and in a colleague.

Attorney General John Ashcroft said the findings raise concerns that a terrorist might have released anthrax. "We don't have enough information to know whether this could be related to terrorism or not," Ashcroft said at a news conference in Washington. He said the probe "could become a clear criminal investigation."

Last week, Tommy G. Thompson, the Health and Human Services secretary, had described the Florida case as an isolated and most likely a naturally occurring event. Public health officials headed the investigation last week, but the FBI took control yesterday.

The colleague, a mailroom worker who was not identified, was reported in good condition at an undisclosed hospital in Miami-Dade County. The Florida health department said the man was being treated for another illness when a nasal swab turned up anthrax spores.

"He hasn't been diagnosed with the disease - there's just a presence of [the spores] in his nostril," said Tim O'Connor, regional spokesman for Florida's health department.

The FBI sealed off the Boca Raton building that houses several supermarket tabloids, including the Sun, where the two men worked. Five hundred workers and recent visitors to the building lined up yesterday at a health center in Palm Beach to be tested for anthrax exposure and receive antibiotics in the event they were exposed.

Researchers also took swabs from numerous locations in the newspaper offices and hope to have the results of those tests back soon, O'Connor said.

Investigators are reviewing medical records at local hospitals to determine if other people reported suspicious symptoms, said Dr. Jean Marie Malecki, director of the Palm Beach County health department.

Bob Stevens, the 63-year-old Sun photo editor who died of anthrax Friday, was the first person diagnosed with inhalation anthrax in the United States since 1978.

The most serious form of anthrax, it first appears as a flu-like illness with fever, muscle aches and fatigue, later giving way to internal bleeding and coma. It is fatal in about 80 percent of the cases, though antibiotics are often successful if used early.

Cutaneous anthrax, which occurs when anthrax enters the body through cuts or sores, is more common and is rarely fatal if treated with antibiotics. Both forms of the disease can be acquired from infected livestock. Anthrax infects sheep, cows and goats and can be spread through the air to people who encounter rotting carcasses or who work with hides or wool.

Stevens' case had raised suspicions of a possible terrorist attack because he did not work with animals and because anthrax has been developed as a biological weapon by Iraq, the Soviet Union and other nations. Inhalation anthrax is considered the most likely form to be spread through bioterrorism because of its lethality and ability to be dispersed widely.

Also, Stevens lived about a mile from an airstrip where suspected hijacker Mohamed Atta rented planes, according to the owner of a flight school there. Several suspected hijackers also visited a crop-dusting business about 40 miles from Stevens' home.

Learning of yesterday's developments, several experts outside the investigation said they were hard-pressed to imagine how the Florida scenario could have resulted naturally. The disease is not spread from person to person, and it was unclear if the men had traveled together to a place where they could have been exposed.

"It is certainly hard to imagine an easily plausible explanation," said Dr. Tom Inglesby, an infectious disease specialist with the Johns Hopkins Center for Civilian Biodefense Studies.

"The source would be an animal product if this were a natural occurrence - either infected wool or some kind of hide or skin. Certainly, there is no previous natural occurrence of anthrax in a non-animal facility."

Bill Patrick, who headed product development in the United States bioweapons program until it was disbanded in 1969, said a person needs to inhale an extremely large dose of anthrax spores to become infected.

It is unlikely, he said, that two people would receive such a large dose - 10,000 spores - unless they were subjected to a deliberate release.

"It's highly presumptive evidence that we have a bioterrorist attack," said Patrick, who is retired from the United States Army Medical Institute of Infectious Disease.

A person could contract anthrax in the wild by stomping on a contaminated animal carcass hard enough to raise a dense cloud of spores, he said. Even if two workers inhaled spores in this manner - a remote possibility - it wouldn't explain how anthrax made its way into the workplace.

Dr. C.J. Peters, former chief of the CDC's special pathogens branch, raised the possibility that investigators are finding anthrax because they are looking for it.

"We don't normally go around culturing people or keyboards for anthrax," said Peters, who heads a new bioterrorism institute at the University of Texas at Galveston. "In this case we have a real motivation to do so."

Lab tests have shown that the strain of anthrax found in Florida responds to antibiotics if they are given early.

"It is certainly not an engineered strain, not a strain like the Russians produced and wrote about," said Dr. Donald A. Henderson, director of the Hopkins bioterrorism institute. "They said they had produced an antibiotic-resistant strain."