Officials from the US Federal Bureau of Investigation (FBI) are draining a pond to search for evidence linked to the 2001 anthrax attacks which left five people dead and caused global panic.
No-one has yet been charged in relation to the anthrax attacks
The pond, in a forest in the town of Frederick in the US state of Maryland, is one of a series that has been searched in recent months by FBI divers following a tip-off in December.
In May the Washington Post newspaper reported that FBI investigators had found equipment in one of the ponds, which they believe could have been used in the anthrax attacks.
The FBI said in a brief statement on Monday that the search was expected to take several weeks but that "there is no indication of any threat to public health or safety".
The Washington Post said that FBI divers searching the ponds had earlier found vials wrapped in plastic and a box similar to an airtight chamber, with built-in gloves used by scientists to handle dangerous biological or chemical agents.
The anthrax attacks caused widespread panic
The findings raised fresh hopes of solving the case, which had seemed to be at a standstill after an 18-month long investigation.
They also led investigators to formulate a new theory regarding the attacks, whereby the perpetrator may have created the laced letters in an airtight chamber submerged underwater to avoid exposing himself to the spores or leaving an evidence trail.
The attacker would then have hidden the evidence in the ponds.
Some have disputed the theory, saying that the suspect could have engineered the letters on land and simply used the pond to dump evidence.
However, scientists told the paper that investigators could test the pond's sediment for traces of the deadly anthrax spores.
FBI officials have long been baffled by the fact that no traces of anthrax have been found anywhere other the places contaminated by the actual letters.
In addition to the five people who died, more than 20 were made ill by letters laced with the lethal form of inhalation anthrax that were sent through the post to various media and political outlets in the US, including the offices of two senators.
The attacks caused global panic, as governments scrambled to assure citizens that they would be protected against any biological or chemical attack.
No-one has ever been arrested in relation to the anthrax attacks.
Attorney General John Ashcroft has mentioned former US army scientist Steven Hatfill as a "person of interest" regarding the case.
However Mr Hatfill, a germ warfare expert who used to work at the Army Medical Institute of Infectious Disease at Fort Detrick - the army base the town of Frederick surrounds - has denied all links to the attacks.
And local law enforcement officials did not link the pond draining to Mr Hatfill.