It was not immediately known what prompted the search, or exactly what investigators were looking for.
"There is no present danger to public health or safety,'' said Joe Parris, FBI supervisory special agent.
CBS News Correspondent Sharyl Attkisson reports the new searches involve three private properties linked to a New York State physician, including a waterfront home in Dover Township, N.J., and two homes in Wellsville, N.Y.
CBS News is not naming the property owner and the FBI isn't calling him a suspect or "person of interest." But agents driving black and white vans removed boxes and bags of possible evidence.
The FBI does confirm the search warrants are being served in connection with the infamous anthrax-mail attacks. Shortly after 9/11, anthrax-laced envelopes were sent to news organizations and government offices, including those of Senate Democratic Leader Tom Daschle of South Dakota and Sen. Patrick Leahy, D-Vt. Five people were killed and 17 injured.
Some of the letters were processed at a postal facility in New Jersey about 35 miles from one of the homes that was searched Thursday.
It's the first time the FBI has combed a private residence in more than two years in the probe. Last month, agents quietly moved into the U.S. Army Medical Research Institute of Infectious Diseases in Ft. Detrick, Md., as part of the probe. Several labs were temporarily shut down.
It's not clear whether all the activity signals new breaks in the unsolved case.
Steven Hatfill, a bioterrorism expert who's never been charged, has been the target of numerous FBI search warrants and around-the-clock surveillance. The FBI even drained a pond looking for evidence linking him to the attacks. Hatfill is now suing the Justice Department saying the whole matter has ruined his reputation and job prospects.
One source seemed to downplay Thursday's searches, calling it a "routine follow-up to eliminate other persons of interest." After three years and more than 5,000 interviews there are -- still no arrest or firm answers to the puzzling anthrax mystery.