BBC News Online: World: Americas

Anthrax cases baffle investigators

Hundreds of anthrax tests are being carried out

Meanwhile in New York, officials are still trying to discover how hospital worker Kathy Nguyen contracted inhalation anthrax to become the latest victim of the disease.

Four people have died of the disease, with another 13 confirmed cases.

Initial tests at Ms Nguyen's Bronx home and the Manhattan hospital where she worked have proved negative.

Terrorism

President Bush has described the anthrax threat as a second wave of terrorist attacks, following the plane hijackings on 11 September.

Earlier, the United States Federal Bureau of Investigation (FBI) has appealed again to the public for help in tracking down those responsible for the anthrax attacks in the United States.

The FBI's director, Robert Mueller, said he was disappointed at the response so far despite the $1m reward for information, and urged the public to provide leads.

Culprits unknown

Mr Mueller said the agency was aggressively investigating the confirmed cases of anthrax, but admitted it was no closer to finding the culprit.

"We have not received as many tips or leads as we would like," he said.

"Despite speculation about the possible source of the anthrax and the motive for the attacks, nothing yet has been ruled out," he said.

"And so today I am calling upon all of you in the news organisations to basically speak for us, to ask our fellow citizens to join us in trying to bring leads to the front that will help us solve both the anthrax investigation, but also the 11 September hijacking investigations."