The F.B.I. began draining the pond near Frederick, Maryland on Monday.
This winter, investigators found a plastic box in the pond that officials say could have been used to prepare the letters.
The box, like this one, had hand-sized holes cut in it. Investigators say it may have served as a makeshift glovebox, like those which scientists use to safely handle dangerous materials.
The pond is just a few miles from the U.S. Army's main biological weapons research laboratory at Fort Detrick.
It is also close to the apartment of former Fort Detrick employee and biological weapons expert Steven Hatfill.
Mr. Hatfill has not been named a suspect in the anthrax letter attacks. But the F.B.I. describes him as a person of interest in their investigation.
Mr. Hatfill's spokesman, Pat Clawson, says his client had nothing to do with the anthrax letters.
PAT CLAWSON, SPOKESMAN
"They can drain the Pacific Ocean and they're not going to find any evidence that Steve Hatfill was the anthrax killer because he's had no involvement whatsoever."
Investigators say it may take as long as a month to drain the pond before investigators can scour the bottom for evidence.