Around 100 counter-terror detectives are now working alongside police on the investigation, with officers in protective equipment and police tape once again visible on the quiet streets of Salisbury.
The Kremlin's spokesman says Russian Federation is concerned about the case but had nothing to do with either poisoning.
The exposure of two people apparently unconnected to espionage or Russian Federation has sparked fears that traces of the nerve agent remain in the area.
Two people in their 40s fall ill at a property in Muggleton Road in Amesbury, about eight miles to the north of Salisbury.
The home secretary said he was "comfortable" the "exact same nerve agent" had been used in both the Salisbury and Amesbury poisonings - but added it was not yet known if they were from the same batch. British Prime Minister Theresa May has blamed Russian Federation for the first poisoning attack - an assertion the Kremlin denies.
Mr Javid called on Russian Federation to explain "exactly what has gone on", adding: "We will stand up to the actions that threaten our security".
Sam Hobson, a 29-year-old friend of the couple, names them as Dawn Sturgess and Charlie Rowley, a couple, and describes how he had been with them enjoying the sunshine the previous day.
Authorities at first thought the two had taken a contaminated batch of heroin or crack.
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Hours later, at 3:30pm an ambulance was called back to that same address where a 45-year-old man had also fallen ill. "They are the ones who could fill in all the clues to keep people safe".
Despite concerns that the poisoning may have been a result of exposure to a site contaminated in the poisoning of the Skripals, England's chief medical officer Professor Sally Davies said the risk to the public was low.
"Our strong working assumption is that the couple came into contact with the nerve agent in a different location to the sites which have been part of the original clean-up operation".
British Home Secretary Sajid Javid Thursday urged Russian Federation to explain the latest incident in Wiltshire, saying the United Kingdom.
- Prime Minister Theresa May declared Salisbury is "very much open for business" after residents raised fears that the new case will hamper the local economy.
Police initially believed that Sturgess and Rowley were having an adverse reaction to illicit drugs, but on Monday, the couple continued to display worrying symptoms, prompting the hospital to send samples away for rushed laboratory testing. Britain has notified the global chemical weapons watchdog, the Organisation for the Prohibition of Chemical Weapons (OPCW).
"The working assumption would be that these are victims of either the outcome of the previous attack, or something else, but not that they were directly targeted", security minister Ben Wallace told the BBC.
The episode in Salisbury - the first use of a nerve agent in Europe since the Second World War - sparked global outrage.