A second Florida man has been exposed to the rare and extremely deadly anthrax bacterium, and U.S. officials said Monday that they were vigorously investigating the incident as a potential criminal act.

One law enforcement source said they are considering the possibility of terrorism.

The man, mail room employee Ernesto Blanco, was a co-worker of Bob Stevens, photo editor at the Sun tabloid newspaper, who died Friday of the first case of inhaled anthrax seen in the United States in a quarter-century.

Over the weekend, authorities also found traces of anthrax bacteria on the keyboard of Stevens' computer at work.

In Virginia, state and local officials said late Monday night that they were monitoring a possible case of anthrax in a northern Virginia man whose job may have brought him into contact with the company where the two Florida men worked.

The Virginia man entered the emergency room of Prince William Hospital in Manassas on Monday, complaining of flu-like symptoms, it was reported in today's Washington Post. Medical personnel quickly performed tests to determine whether he had anthrax, meningitis or another disease.

Officials of the Centers for Disease Control and Prevention monitored the preliminary investigation. A germ culture from the man was transported Monday night by a state police trooper to a state government laboratory in Richmond, officials said.

Earlier Monday, the Florida newspaper's office building in this upscale Palm Beach County community was quarantined, the company's 300-member staff was undergoing testing at a nearby health clinic and more than 50 investigators from federal and state agencies were working on the case.

"We regard this as an investigation which could become a clear criminal investigation. And we are pursuing this with all the dispatch and care that's appropriate," Atty. Gen. John Ashcroft said in Washington.

At this point, Ashcroft emphasized, "we don't have enough information to know whether this could be related to terrorism or not."

There were indications that the probe was also taking into account the often lurid nature of the Sun's news coverage, and the possibility that the weekly might have provoked a reader or a former employee.

A law enforcement official who spoke on condition of anonymity said Ashcroft became particularly concerned after learning that Stevens and Blanco worked 17 miles from an airport in Lantana where Mohamed Atta, the apparent ringleader of the Sept. 11 hijackings, rented a small plane for three days in mid-August. Moreover, Stevens, 63, was a resident of Lantana.

Asked Monday whether Atta was considered a suspect in the mysterious occurrences of anthrax, Ashcroft said: "We haven't ruled out anything at this time."

Frank Pinela, spokesman for Florida's Department of Health, said that anthrax spores had been detected in Blanco's nasal cavities but that he did not contract the disease. The germs turned up on cotton test swabs Sunday after Blanco, who is in his 70s, was admitted to a hospital with pneumonia.

Health officials said Blanco was in stable condition and being treated with antibiotics Monday at a hospital in the Miami area. He was expected to survive, FBI spokeswoman Judy Orihuela said.

"We don't know the cause. We're investigating at this time," Pinela said. "We're leaving no stone unturned."

The offices of American Media, which publishes the Sun, were closed at the request of authorities Sunday night. The next day, the three-story building was cordoned off as FBI agents and public health officials scoured the premises.

Florida Health Secretary John Agwunobi urged employees, and anyone who had spent an hour or more inside the building since Aug. 1, to go to his department's annex in Delray Beach for testing and a preventive dose of antibiotics.

Nervous employees waited outside the clinic Monday to have swabs inserted in their noses and the bits of cotton sent to Miami for testing.