The woman who contracted the skin form of anthrax is an assistant to NBC anchorman Tom Brokaw, authorities said Friday.

The NBC employee is expected to recover. Authorities said she was possibly infected by the bacterium when she opened an envelope containing a suspicious powder sent to Brokaw two weeks after the Sept. 11 attacks.

Video: HHS Sec. Thompson on NBC Anthrax Exposure
Video: Attorney General Ashcroft on Anthrax Investigation

The letter was postmarked from Florida, as was another suspicious letter received at the offices of the New York Times, according to officials.

Officials added there was no known connection to either the Sept. 11 attacks or the far more dangerous inhaled form of anthrax that killed a supermarket tabloid editor in Florida last week.

In Nevada, a letter sent to a Microsoft office in Reno originally tested positive for anthrax before a second test indicated it probably did not contain the bacterium, state health officials said. The letter had been sent to the Microsoft Licensing Inc. office from Malaysia.

A third test is planned, and the FBI is involved, according to state epidemiologist Dr. Randy Todd. Results of that test should be available Saturday.

A federal criminal investigation has been launched to learn the origin of the letter sent to the New York Times, and health officials planned to re-test powder to see if it contained the germ. Initial tests on a small sample revealed no sign of anthrax. 

A city still reeling from the World Trade Center disaster was shaken by word of the disease. News organizations increased mailroom security nationwide. And the postmaster general advised everyone to be on the lookout for suspicious letters and packages.

President Bush said the government was working to ensure public safety.

"The American people need to go about their lives. We cannot let terrorists lock our country down," Bush said, addressing the anthrax case at a White House event celebrating Hispanic heritage. "They will not take this country down."

The nation's fourth anthrax case in a week was reported early Friday by the federal Centers for Disease Control and Prevention after tests were completed on a skin sample from the victim. Further tests are under way.

"The amount we got was so small we are very being cautious about interpreting," CDC Deputy Director David Fleming said.

The CDC said it is possible the NBC employee contracted anthrax from something other than the envelope.

Part of the 70-story GE Building in Rockefeller Center, which houses Brokaw's "Nightly News," "Saturday Night Live" and "Late Night with Conan O'Brien" was evacuated.

On condition of anonymity, NBC officials confirmed that the employee who tested positive was a Brokaw assistant. One official said that Brokaw, an 18-year veteran of NBC's evening newscasts, was being tested for anthrax.

"Living in New York and working in this building for this company, you're already on edge," said Brian Rolapp, 29, a business development manager for NBC. "I think everyone is a little startled that it's this close to home."

A few blocks away, two floors of The New York Times building were evacuated Friday after Judith Miller, a reporter who co-wrote a recent best seller on bioterrorism, opened a letter containing a powder-like substance a spokeswoman said smelled like talcum powder.

Executive Editor Howell Raines said initial tests showed that the powder did not pose any immediate problem, and air tests for radioactive and chemical substances were negative.

The Associated Press, located across the street from NBC, temporarily closed its mailroom, as did CBS. ABC halted internal mail delivery in New York and Washington pending a security evaluation, while CNN said it had closed its mailrooms in New York, Washington and Atlanta.

Time Inc. and Newsweek suspended mail delivery in their New York headquarters. The Wall Street Journal told employees worldwide not to open packages or letters that are not specifically addressed to them.

In Los Angeles, an anthrax scare on the third floor of the Los Angeles Times Building caused employees to remain in the building for several hours on Friday night. A "powder-like substance" was tested and determined to be harmless, according to authorities.

The skin and inhaled forms of anthrax are caused by the same bacterium. The only difference is whether the microscopic spores enter the skin through a cut or are inhaled into the lungs. It takes more than 8,000 spores to cause the inhalation form of anthrax. Neither form of anthrax can be spread directly from person to person.

When caught through the skin, anthrax is a much less serious disease. The first symptoms are reddish-black sores on the skin. The disease is easily cured if it is caught at that point and treated with antibiotics. Even without treatment, cutaneous anthrax is lethal in only one case out of 20.

Dr. Scott Lillibridge, the bioterrorism chief for the U.S. Health and Human Services Department, said the NBC employee is thought to have come in contact with the envelope on Sept. 25. Three days later, she noticed a dark-colored lesion, Lillibridge said, and on Oct. 1 began taking the antibiotic Cipro for another infection.

Mayor Rudolph Giuliani said all network employees exposed to the powder will be tested for anthrax and treated with Cipro.

"People should not overreact to this," Giuliani said. "Much of this is being done to allay people's fears."

Last Friday, a photo editor for The Sun supermarket tabloid in Boca Raton, Fla., died of the more serious inhaled form of anthrax. The American Media building where Bob Stevens, 63, worked was sealed off after anthrax was found on his keyboard.

Traces of anthrax were later found in the mailroom. Two other employees turned out to have anthrax in their nasal passages, although neither has developed the disease. Both are taking antibiotics, and one has returned to work.

In Florida, FBI agent Hector Pesquera said test results of 965 people who were in the building recently found no new infections. A few test results were still pending. Pesquera said investigators are still trying to determine how the anthrax got into the building.

Fifteen clerks who worked in the South Florida post office that handled American Media's mail were also tested, a union official said.

Also on Friday, two corporate jets were ordered to land by the Federal Aviation Administration after an air traffic controller heard the word "anthrax" used on a cockpit radio frequency. One jet landed in Austin, Texas, and the other in Tulsa, Oklahoma.

The jet ordered to land in Austin was checked by the FBI and allowed to continue, while Tulsa was the intended final destination for the other plane.

Separately, Health and Human Services Secretary Tommy Thompson said Friday that the Bush administration planned to ask Congress for $643 million to expand the nation's capability to handle biological attacks.

The money will be used to increase supplies of pharmaceutical drugs and anthrax antibiotics, as well as to fortify health systems throughout the country, Thompson told newspaper editors in Wisconsin via video from Washington.

The Associated Press contributed to this report.