Erin O'Connor, 38, an assistant to NBC anchor Tom Brokaw, opened a letter to her boss containing a mysterious white powder on Sept. 25.

"The letter was given over to the FBI, but they didn't seem to take it seriously," her mother, Eileen, told The Post yesterday.

Officials initially thought Erin contracted the disease by handling that letter, but Mayor Giuliani said health officials determined she became ill by handling a letter mailed to the network earlier.

Eileen O'Connor said that on Sept. 28, Erin, who has worked for Brokaw for 10 years, developed a temperature and felt nauseous, her mother said.

"Then, what appeared to be a big bug bite appeared on her shoulder," she said.

Erin went to an infectious-diseases clinic in Manhattan and was given the antibiotic Cipro.

"A battery of tests were done, but the doctors did not immediately know what was wrong," Eileen said.

After a week, the "bug bite" turned into a big, black scab.

"Erin was very frightened and went to another doctor to get a second opinion. She was very concerned, and they did more tests," her mother said.

A skin biopsy was taken, and early Friday the results came back - Erin had the skin form of anthrax, which is less severe than the inhaled form.

Eileen O'Connor said her daughter "is very traumatized."

Her husband, NYPD cop John Corcoran, is "trying to calm her down, but it is very hard in the circumstances."