U.S. officials are investigating several cases of suspicious powder and testing them for the possible presence of anthrax bacteria. Some tests are proving negative, but at least two new findings of the anthrax bacteria have been made. Officials in New York and Nevada offered new information on those cases Saturday, and a spokesman for a publishing company in Boca Raton, Florida, where three workers contracted the inhaled form of anthrax, said five more of its employees were found to have been exposed to anthrax bacteria.

New York Mayor Rudolph Giuliani told reporters a letter sent September 18 from New Jersey to NBC anchorman Tom Brokaw has tested positive for anthrax. He says that is the letter that infected Mr. Brokaw's assistant. The woman contracted anthrax through a cut in her skin. She is being treated with antibiotics and is expected to recover.

The FBI chief in New York, Barry Mawn, says investigators had thought a letter sent September 25th caused the infection. Officials were following up on that Friday when the second, earlier letter was discovered.

"During the course of that, we were provided information for the first time that there was another letter, threatening in nature," said Mr. Mawn. "We retrieved that letter, and that's the one that we're speaking about." Mr. Mawn added that the letter contained an unspecified threat and a brown granular substance.

The NBC employee is the fourth confirmed case of anthrax in the United States.

Nevada Governor Kenny Guinn said Saturday a letter sent from Malaysia to a Microsoft office in Reno has tested positive for traces of anthrax. But he says there are no known cases of infection stemming from that letter, adding it is not yet known if the strain is one that causes a threat to people's health.

New York Mayor Giuliani says Americans have learned a lot about anthrax in recent days. But he cautioned people not to panic about the outbreaks. "It is not contagious," he explained. "It can be treated and cured. The reality is the treatment for it that exists is effective. So people shouldn't be overwrought about it or nervous about it. It is something that can be dealt with."

Meanwhile, a powder in an envelope sent to a New York Times reporter has tested negative for anthrax. And the Federal Aviation Administration grounded a domestic flight Saturday after an unidentified powder was found on board. It was later determined to be non-toxic.