Second NBC anthrax case?

NEW YORK (CNN) -- Mayor Rudy Giuliani said Saturday authorities have found the source of the anthrax that infected an NBC employee, and New York City Health Department officials are investigating another possible infection in another NBC News employee.

Giuliani said early suspicions had focused on a threatening letter containing a powder addressed to anchor Tom Brokaw and postmarked September 20. But three tests of the powder proved negative, though the employee tested positive for the cutaneous (skin) form of the illness.

A second threatening letter, postmarked September 18 and also containing a powder, was found Friday and appears to be the source of the infection of the woman. She is being treated with antibiotics and is expected to recover.

"We believe we found the letter that explains the reason why the woman there was infected, and the original powder that was examined turned out to be negative," Giuliani told reporters.

The second letter was postmarked in Trenton, New Jersey, and was also addressed to Brokaw, he said. It tested positive Friday night for anthrax.

The September 20 letter was postmarked from St. Petersburg, Florida.

Meanwhile, a statement from the New York City Department of Health said the department is investigating a second possible case of cutaneous anthrax in another NBC News employee.

Two other envelopes containing threatening letters and powder were postmarked in St. Petersburg: one was received by a reporter at The New York Times and another was received by The St. Petersburg Times. Both of them were postmarked October 5.

None of the tests on the powder from letters postmarked from St. Petersburg has proven positive, Giuliani said.

The mayor attempted to calm fears that a wave of illness was being spread through the city. "Right now, we have one case that has been dealt with," he said, referring to the NBC News employee.

"But beyond that, there's no indication of any kind of spread of anthrax of any kind," he said.

At NBC, 358 people have been tested. "We're going to have to go back and see if there are more people who need to be tested," he said.

Barry Mawn, head of the FBI's New York office, said the envelope sent to NBC containing the anthrax was white, contained a threatening letter whose contents he would not describe, and bore no return address.

Most of the powder inside had been thrown out. "Our understanding is that it was a brown, granular substance," Mawn said. Nevertheless, enough of it remained to test positive for anthrax, he said.

"We've identified the missing link," said NBC Chairman Bob Wright. "The actual cause of the anthrax which has created this whole situation."

He said it was not unusual for NBC personnel to receive threatening letters and that the letter containing the anthrax was kept inside an envelope containing other threatening letters. "It's fortunate that this letter was saved," he added.

Meanwhile, NBC personnel can get back to work, said Dr. Steven Ostroff of the Atlanta-based Centers for Disease Control and Prevention.

No one else who works in the building has reported symptoms of the disease, he noted.

"The public health risk associated with that building is pretty close to negligible, and our recommendation is the employees of NBC need to get back to their business," he said.