The police said that they had requested military assistance to "remove a number of vehicles and objects from the scene".
Former double agent Sergei Skripal, 66, and his daughter Yulia, 33, have been in hospital since they were found unconscious on Sunday on a bench outside a shopping centre in the quiet cathedral city of Salisbury.
The Times newspaper said police were probing whether Skripal's daughter, who arrived in Britain from Moscow last week, may have inadvertently brought in the nerve agent as a gift.
Only DS Bailey and the Skripals, who are both critically ill, remain in hospital.
The Ministry of Defence has sent in 180 specialist personnel and equipment to support the police investigation.
Sites that have been cordoned off include the city center bench where the two were found, a pub and restaurant they had visited earlier, Skripal's home, the graves of his wife, Lyudmila, and son, Alexander, and an industrial vehicle park.
Britain has said it will respond robustly if evidence shows Russian Federation was behind the attempted murder.
Home secretary Amber Rudd will chair the second Cobra meeting in a week after the investigation into the attempted murder of an ex-Russian spy and his daughter continues. He said Russian Federation was ready to assist "any investigation" but that it was "not necessary to hurl unfounded accusations on TV".
Russia's foreign minister says the country could help Britain investigate the poisoning there of an ex-Russian spy, but expressed resentment of suspicions cast on Moscow.
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On Saturday, investigators dressed in protective suits were seen around her grave with a brightly coloured gurney placed nearby, while workers erected a barrier at the entrance to the cemetery where Lyudmila was buried following her death in 2012.
Noting it was "highly likely" that the officer, Bailey, was exposed to the same nerve agent as the Russian pair, she said Britain "will act without hesitation as the facts become clearer".
In a statement on Wednesday evening, Mark Rowley, the head of the counterterrorism division at London's Metropolitan police, described the attack as a "major incident" and said hundreds of police officers, forensic experts and government chemical analysts were involved in the inquiry.
With police also hurt in the attack, pressure is intensifying on Prime Minister Theresa May to find and punish the culprits.
Police say they know the nerve agent used in the attack, but have declined to say what it was or how they suspect it was administered.
The cause is unclear: Some reports say he died in a auto accident while on holiday in Russian Federation, others that he died of liver failure. He was convicted in Russian Federation of spying for Britain before he was granted refuge in the UK after a high-profile spy swap in 2010 between the United States and Russian Federation.
A detailed United Kingdom inquiry later concluded that Russian President Vladimir Putin probably approved the operation by Russian agents to kill Litvinenko.
The attack has been likened in Britain to the assassination of ex-KGB agent Alexander Litvinenko, a critic of Russian President Vladimir Putin, who died in London in 2006 after drinking green tea laced with radioactive polonium-210.
"Of course, if action needs to be taken, then the government will do that".