The FBI is investigating the anthrax outbreak
A chemist in the town where the outbreak was discovered told The New York Times newspaper he was selling Ciproflaocin antibiotic tablets at a rate more than 20 times higher than usual as people "panicked".
Computer security experts, for their part, are expected to warn Congress on Wednesday of the possibility of a terrorist attack on systems responsible for communications, power or water.
"To put it simply, we need more people to be doing more creative thinking about computer security. That's what our adversaries are doing," the chairman of the House Science Committee, Sherwood Boehlert, told the Associated Press.
Ongoing investigation
Results of the FBI and Centers for Disease Control investigation into the anthrax outbreak will not be available for several days.
While the FBI says the incident is probably the result of foul play, it is keen to play down the possibility of a link with those responsible for the attacks on New York and the Pentagon last month.
There is still no evidence that the outbreak was the result of a bio-terrorist attack, and correspondents say the investigation appears to be going in other directions.
Federal investigators are comparing the strains of anthrax found in Florida against known strains kept at a national respository in Arizona.
Initial tests of the anthrax found in Florida match strains produced for research purposes in the 1950s at a laboratory in the state of Iowa.
Although those strains may have been widely distributed, investigators say that if the match is confirmed, it would rule out the possibility that the men contracted anthrax by natural or accidental causes.
Meanwhile, hundreds of employees of American Media, the tabloid newspaper publisher where the outbreak was discovered, have been tested for anthrax.
No new cases have been found.
Envelopes containing a white powder have been mailed to about a dozen locations around Florida, but a state health official told The New York Times that they were probably a hoax.
In the wake of US retaliatory strikes against Afghanistan which began on Sunday, government officials have urged police and private security agencies to take extra precautions.
"I encourage all Americans to have a heightened sense of awareness of their surroundings," Attorney General John Ashcroft said.
"I ask them to report suspicious activity to our partners in law enforcement. Every American should be vigilant."
The government officially stands at a Level 3 state of alert, indicating that there is potential for a terrorist attack but no specific and credible threat.
Incidents
A number of incidents since the 11 September attacks have kept Americans on edge.
A subway station near Washington DC was shut down for six hours on Tuesday when a man sprayed a substance into the air and scuffled with police.
Tests showed that the substance was a cleaning agent, but passengers on the train reported feeling ill after the incident.
Local officials say their syptoms could be the result of pepper spray used by officers in subduing Kenneth Ranger, 23, who has been charged with attempted murder of a police officer.
A day earlier, passengers and pilots on an American Airlines flight subdued a passenger who broke into the cockpit during a flight from Los Angeles to Chicago.
The plane, with 162 people aboard, landed safely.
Edward Coburn, 31, has been charged with interfering with a flight crew.