Officials have ruled out direct links to terror attacks
It now says that foul play is the most likely explanation for two anthrax cases, one of which proved fatal.
Hundreds of people employed at American Media Inc have been queuing to undergo tests for the disease.
US officials have ruled out any direct link with the terrorist attacks on 11 September.
Bacteria on keyboard
The FBI has sealed off the office block that houses American Media Inc in Boca Raton. The company publishes the National Enquirer among other titles.
The source of the infections has yet to be identified.
Public health officials began a search of Stevens' home and car to try to find the source of the anthrax spores.
They eventually found traces of the bacterium on his computer keyboard at work.
Robert Stevens, 63, had been admitted to the hospital suffering from confusion and vomiting and his condition quickly deteriorated. He died from the rare respiratory form of the disease.
Mr Stevens' case was the first of respiratory anthrax in the US since 1978.
'One in a billion'
Dr Landis Crockett, director of disease control for the Florida Department of Health, earlier said the chances of two naturally-occurring anthrax cases in such close proximity were "one in a billion".
US Attorney General John Ashcroft said on Monday that the FBI was taking the case "very seriously".
He said that he could not rule out the possibility of a bioterrorist attack until further tests were carried out.
Other experts are also convinced that the disease has been spread deliberately.
Dr Kenneth Alibek, a bioweapons specialist who spent years growing anthrax and other diseases for the USSR prior to moving to the US, told the BBC's Today programme that an innocent source was "absolutely impossible".
He said: "It's obviously not a natural case, not a natural outbreak - it was a deliberate outbreak.
"It is difficult to find out how this act was planned - but somebody did it."
Officials have stressed that anthrax is not contagious. There is a delay between infection with anthrax and the appearance of symptoms.
Second man
The second man to be infected with the potentially deadly anthrax bacteria, 73-year-old Ernesto Blanco, is a former colleague of the man who died from the disease on Friday.
Mr Blanco has not developed any symptoms of anthrax infection but a swab taken from his nose has tested positive for the bacteria which cause the illness.
He is being watched in hospital, and given antibiotics as a precaution.
Reports of a third case in Virginia were played down by a doctor involved in the case.
The man involved had worked as a contractor in the America Media Inc building and was taken to hospital on Monday. Tests for anthrax have so far proved negative.