The investigation is "highly likely" to take many months and, according to the Metropolitan Police, there is currently no timescale on when the searches will conclude.
Sergei Skripal, a 66-year-old British national, and his 33-year-old daughter Yulia Skripal, a Russian citizen, remain in critical condition in hospital after being exposed to a nerve agent in a targeted attack.
Wiltshire Police's Detective Sergeant Nick Bailey, who was among those first to respond to the incident, was also taken ill and remains in a serious but stable condition in hospital.
The pub, which is part of the Greene King estate, was sealed off shortly after the victims were found unconscious on a bench nearby.
"Specialist search officers wearing protective equipment continue to carry out a meticulous, systematic search for evidence to support the investigation," said the force.
"This is an extremely challenging investigation and police and partners continue to manage a number of unique and difficult issues.
"Around 250 counter-terrorism detectives continue to work round the clock, supported by a full range of experts and partners.
"Officers continue to trawl through 4,000 hours of CCTV and examine nearly 800 exhibits that have been seized. Around 400 witness statements have been taken and many more will follow in the coming days and weeks."
This week reports that the cordon surrounding the Mill pub is looking "more permanent" with the arrival of steel barriers have surfaced.
Last week, Met Assistant Commissioner Neil Basu said: "The public will continue to see a lot of police activity in and around the city, including new cordons being established, while existing cordons may remain in place for some time.
"The number of scenes and their locations is likely to change over the coming days. This is nothing for the public to be alarmed about, but is necessary as part of this major investigation by the counter-terrorism policing network.
"We are prioritising different scenes at different times, depending on where the evidence takes us and the latest scientific advice, while managing the welfare of those attending the scenes."
Earlier this month, customers of the pub were advised to wash their possessions after traces of nerve agents were discovered.
The Morning Advertiser has contacted both Greene King and counter-terrorism police for further comment, but did not receive a reply before publishing.