NEW YORK - Experts are conducting environmental tests and a criminal investigation is underway at the CBS Broadcast Center, where a woman who works in anchorman Dan Rather's office was diagnosed with the skin form of anthrax.

The diagnosis, announced Thursday, was the third involving a major television network.

The CBS employee, Claire Fletcher, is being treated but has continued to work, and network officials said on Thursday that she feels fine. Rather said his colleague doesn't remember receiving any suspicious packages.

The source of the bacteria was still being sought, but city Health Commissioner Neal Cohen said "it makes sense" to suspect that it was delivered in an envelope. Cohen noted that Fletcher works with mail and her case resembles an anthrax infection at NBC headquarters in New York.

Officials said there was no danger to employees or the public.

"Our biggest problem today is not anthrax," Rather said. "Our biggest problem is fear. ... We are resolute, we will not flinch, we will not bend, we will not swerve."

Mayor Rudolph Giuliani, trying to ease public fears, emphasized that 17 days had passed since the appearance of Fletcher's symptoms _ a slight swelling in her cheek _ and that no one else at CBS had been affected.

He also pointed out that New York's two other skin anthrax victims _ an assistant to NBC anchorman Tom Brokaw and a baby whose mother had taken him to ABC News headquarters _ were recovering.

"This all supports the conclusion you would draw from there being no other people with symptoms, and that is that there has been no spread," Giuliani said.

The woman's father, Bernard Fletcher, was interviewed at his home in England.

"Her main reaction is she's a bit annoyed because the press in Manhattan have been camped out on her doorstep and so she's had to move out of her flat," he said. "Obviously she was worried about the disease, but she's been assured she will make a full recovery."

Officials said Claire Fletcher, 27, began treatment Oct. 4 by taking penicillin. When the NBC case became known, she was tested for anthrax and later had a biopsy, which came back positive for anthrax early Thursday.

Rather said Fletcher is a triathlete and, "in addition to working every day, she has worked out every day except one," despite the disease.

Three other people in the United States have been infected with anthrax, including a Florida man who died, his colleague, and a New Jersey postal worker who may have handled letters sent to Brokaw and to Senate Majority Leader Tom Daschle.

More than 30 other people in Florida, Washington and New York have tested positive for anthrax exposure.

Rather said he had no plans to be tested and was not taking medication. Brokaw has said he is taking antibiotics.

The CBS case to light came a day after samples taken from Gov. George Pataki's New York City office tested positive for anthrax bacteria. No employees there have tested positive for the disease.