BOCA RATON, Fla. (AP) A third person who worked at a Florida company has tested positive for anthrax and the case has become the subject of a federal criminal investigation, authorities said yesterday.

The 35-year-old woman, whose name wasn"t disclosed, was hospitalized after a swab of her nasal passages found traces of anthrax. She is being treated with antibiotics and her condition was not immediately known.

FBI agent Hector Pesquera said the anthrax contamination is limited to the Boca Raton headquarters of supermarket tabloid publisher American Media. Anthrax killed a tabloid employee last week and found its way into the nose of mailroom co-worker.

U.S. Attorney Guy Lewis said the investigation would focus on how the anthrax got into the building and why.

Public health officials stressed that there is no public health threat from the anthrax, but the case has heightened fears of a biological attack. The statement from Lewis was the most declarative from federal authorities so far that the anthrax was the result of a criminal act.

Pesquera said authorities had no evidence the anthrax was created by a terrorist group and cautioned that "this is not a time for premature conclusions and inaccurate reporting."

Sun tabloid photo editor Robert Stevens died Friday of inhaled anthrax, a rare and particularly lethal form of the disease. Co-worker Ernesto Blanco has been in a Miami hospital since Monday after anthrax spores were found in his nose. He was in good condition.

Health investigators shut down the American Media building after finding traces of anthrax on the computer keyboard used by Stevens. Pesquera said the latest victim worked in the general area of the other victims.

Florida health officials have said the bacteria in Stevens" blood responded to antibiotics, suggesting it was a naturally occurring strain rather than a laboratory-altered one.

A law enforcement official said on condition of anonymity that preliminary work on the anthrax that killed Stevens has found a possible match to a laboratory strain first isolated in Iowa. However, further tests are being done.

Authorities said the latest victim was one of more than 1,000 people who have been tested by health officials for the presence of anthrax. Most have recently been inside the AMI building and most are still waiting for test results. Many were given supplies of antibiotics and told to come back for more tests later.