Frederick, Md. ? The FBI began draining a pond Monday in a search for evidence that the person who carried out the deadly anthrax-by-mail attacks in 2001 filled the envelopes with the deadly spores under water for his own protection.

The draining of the 1-acre pond in the Frederick Municipal Forest is expected to take three to four weeks. The pond is 4 to 5 feet deep.

The FBI said in a statement that its agents and postal agents were conducting “searches related to the investigation of the origin of the anthrax-laced letters.”

The attacks killed five people and sickened 17 others who were infected by anthrax bacteria sent through the mail.

The pond is eight miles from the Army Medical Research Institute of Infectious Diseases at Fort Detrick, the primary custodian of the strain of anthrax found in envelopes sent to the victims.

Dr. Steven Hatfill, a bioterrorism expert who formerly worked as a researcher at the institute, has been described as a “person of interest” in the investigation. His apartment was next to Fort Detrick.

Hatfill has denied any involvement in the attacks.

The pond is one of 10 searched by divers in December and January after the FBI reportedly received a tip that someone may have used the spring-fed reservoir to assemble anthrax-filled envelopes using equipment submerged in water.

The Washington Post first reported May 11 that items recovered from the pond included a clear box with holes that could accommodate gloves to protect a user. Also recovered were vials wrapped in plastic.

Several FBI and Justice Department officials told The Associated Press, speaking on condition on anonymity, that investigators have a theory that someone could have used these items to safely place the anthrax in envelopes.

Testing of the items has not produced definitive evidence of anthrax contamination, these officials said. The pond is being drained to find out if anything else is there — especially at the bottom of the pond.