About 180 troops have been sent to the southwestern English town to remove vehicles involved in the incident as well as objects that could have been contaminated.
The attack is being treated as attempted murder.
Almost 200 troops from the British Army, the Royal Air Force, and the Royal Marines were deployed on Friday to remove contaminated items from the shopping mall in Salisbury, England, where ex-Russian spy Sergei Skripal and his daughter Yulina were poisoned by a nerve agent.
The figure includes members of the public and emergency staff, some of whom have had blood tests as well as receiving support and advice.
Former Metropolitan Police Commissioner Lord Blair said the "extraordinary attack in Salisbury" is a good reason to investigate whether there is a pattern of former British intelligence collaborators dying in the UK.
TRT World's Sarah Morice has more.
Former Russian spy Sergei Skripal, 66, and his daughter Yulia, 33, were still hospitalized in "very serious condition" after being poisoned earlier this week, UK Home Secretary Amber Rudd said as she visited Salisbury on Friday.
A police officer, one of the first responders to the incident, also fell seriously ill at the scene.
He told broadcaster ITV News that 21 people had attended hospital with possible poisoning symptoms, including the police officer and the Skripals.
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 past year after reportedly dying of liver problems.
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He said they have agreed to stop any dissent immediately and not to allow the country's diversity to kill their nation. US Secretary of State Rex Tillerson has cancelled scheduled events in Kenya on Saturday because he is feeling poorly.
Police have cordoned off the graves of his wife and son, who died since his move to England, suggesting that the bodies could be disinterred for examination.
Mr Seely, who is also a former Soviet Union reporter for The Times, said the investigation needs to be balanced and facts need to be established before fingers are pointed.
Meanwhile, Russia's foreign minister expressed resentment at suggestions Moscow was behind the attack and said the Kremlin 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".
Skripal was a former colonel in Russia's military intelligence who was jailed in his country for betraying agents to Britain's MI6 secret service.
Skripal and an another a man named Igor Sutyagin were flown into the RAF base in Brize Norton in Oxfordshire on July 9 2010.
"Once these are established, then and only then, will an appropriate and proportionate course of action be taken".
Both denied responsibility and Russian Federation has refused to extradite them. However, British Foreign Secretary urged "not to jump to conclusions".
Russian Foreign Minister Sergei Lavrov dismissed Britain's warnings of retaliation were propaganda and not serious.
Analyst Mathieu Boulegue of the Chatham House think tank said it would be "extremely difficult" to pinpoint who committed the attack, and that the response could be a lot more complicated if it turned out that internal factions were responsible.
Investigators are said to be probing whether Yulia could have brought the toxin with her from Russian Federation as a present from friends for her father.