A man in his 60s and a women in her 30s remain in a critical, but stable condition in intensive care after being exposed to a nerve agent.
The attack on Skripal and his daughter is reminiscent of the case of another Russian spy, Alexander Litvinenko, who was poisoned to death in London by Kremlin-linked agents in 2006.
About 180 military personnel have also been called upon by the police to remove evidence in Salisbury potentially contaminated by a deadly nerve agent.
However, the Associated Press quotes the Wiltshire county acting police chief as saying that "around 21 people" have been treated and that "a number" received "blood tests, support and advice".
The Russian government has denied any involvement in the Litvinenko killing or the attempted killing of Skripal, a former Russian agent who had served jail time in his homeland for spying for Britain before being freed in a spy swap.
British authorities have not disclosed what nerve agent was involved.
Based on police reports of the Salisbury incident, experts said it is most likely that the toxin was delivered with an aerosol spray or a wipe, and transferred to the skin or clothing of the victims.
On Thursday, officers sealed off the gravestone of Mr Skripal's wife Liudmila, who was buried in 2012, and the memorial stone of his son Alexander, who was cremated past year, as the trail of evidence grew. Health officials insist there has only been a low risk to the public.
Boulegue said the fact that a policeman had also been affected in Sunday's attack on Sergei Skripal and his daughter in Salisbury necessitated "an immediate and strong engagement" by Britain.
Police later said they are treating the incident as "attempted murder".
This latest theory surrounding Skripal's home address comes as the Home Secretary, Amber Rudd, visited Salisbury's Maltings Shopping centre where Sergei and Yulia were found slumped and unconscious on Sunday (4th March). The policeman, who was first to arrive at the site of the assassination, is in serious condition.
"We will do what is appropriate, we will do what is right, if it is proved to be the case that this is state-sponsored", Ms May told ITV News, when asked whether Britain could expel the Russian ambassador over the attack.
The ex-agent was found unconscious on a bench in the English city of Salisbury on Monday alongside his daughter Yulia.
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".
Earlier this year, British government ministers announced they meant to target rich Russians in Britain who couldn't prove where their money came from, and they announced new asset-seizure powers known as unexplained wealth orders. London's financial services industry, which accounts for a significant portion of Britain's GDP, could lose as much as 25 percent of its business when Britain exits the European Union and British bankers and investment managers hope to make up the difference by being even more attractive for foreign investors and the wealthy.
In response to questions over Russia's possible involvement, May has said that "if action needs to be taken then the government will do that". He was sentenced to 13 years in prison in 2006 after a secret trial.
Greene said that "a lot of these things are being done by people operating at sort of an arm's length's distance from the command and control structure".
Moscow has reacted angrily to the accusations it was involved, with Foreign Minister Sergei Lavrov on Friday dismissing them as baseless "propaganda".
The Russian embassy in the United Kingdom, while quoting the version of British authorities negated any Russian links of the spy, in one of the tweets posted on Thursday.