Skripal and Yulia remain in a serious condition along with a police officer who came in contact with the same substance.
Moscow is prepared to cooperate with a British investigation into the death of ex-spy Sergey Skripal, the Russian foreign minister said.
Around 180 troops, including Royal Marines, RAF Regiment troops and chemical warfare specialists, are understood to have been deployed after Scotland Yard requested specialist help to remove vehicles and objects from scenes scattered across the city amid contamination fears.
A police officer critically injured responding to the Salisbury nerve agent attack is in a stable condition, authorities confirmed as the injury toll climbed to 21.
Skripal was convicted by Russian Federation in 2006 of spying for Britain, and was given refuge in the United Kingdom in 2010 as part of a high-profile "spy swap".
Hundreds of investigators, led by counter-terror police, are working to find out who is responsible for what is feared to be a sophisticated plot amid heightened tensions between Britain and Russian Federation.
Nick Bailey is a sergeant for Wiltshire Police, who received an award in 2016 for helping to capture a rapist who was later jailed for more than 14 years.
The Salisbury attack indicates the use of sophisticated chemistry in production of the nerve agent, suggesting the involvement of a state actor, experts said.
Sky News' crime correspondent Martin Brunt said there are suggestions police believe the spy, his daughter and DS Bailey were poisoned at the Russian's home.
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They also sealed off the grave of Skripal's wife, Liudmila, who died in 2012 from cancer, as well as the memorial stone of his son, Alexander, who was cremated a year ago after reportedly dying of liver problems.
Investigators wearing hazmat suits have been seen searching Sergei Skripal's house and auto for evidence, and parts of Salisbury city centre near where the victims collapsed remain cordoned off. "This was attempted murder in the most cruel and public way".
On March 4, the local police said the 66-year-old man and his 33-year-old daughter Yulia were found unconscious on a bench near The Maltings shopping center in Salisbury, following exposure to an unknown nerve agent. They in turn probably exposed a police officer attending the incident who developed less serious symptoms.
Rudd declined to say if she believed Russian Federation was behind the attack, but says Britain will "if it is appropriate, attribute it to somebody".
Those branded enemies of the Russian state have sometimes died mysteriously overseas, and the Skripal case echoes the death of Alexander Litvinenko, a former Russian agent who was poisoned in London in 2006 with radioactive polonium-210.
Public Heath England issued a statement, saying that the police are guarding Skripal's Salisbury residence and a yellow forensic tent has been set up there, as the police are trying to find out whether assailants attacked the double agent with radioactive polonium.
Russian Foreign Minister Sergey Lavrov said Moscow was "ready to consider" lending a hand in any investigation, "whether it's [about] poisoning of some British subjects, whether it's rumors about interference in the US election campaign".
"You can not tolerate a government assassination on British soil - it is absolutely beyond the pale and needs a reaction", he said. Moscow claimed that he had revealed his identity to the British foreign intelligence service - MI6 - in exchange of pecuniary and other benefits. Several British lawmakers immediately accursed the Kremlin of being behind Skripal's poisoning, as well, though investigators say it is still too early to attribute blame.
Russian Foreign Ministry spokeswoman Maria Zakharova said Johnson's comments were "wild".