Inspectors from the world's chemical weapons watchdog have begun examining the nerve agent used to poison ex Russian spy Sergei Skripal and his daughter Yulia.
Experts from the Organisation for the Prohibition of Chemical Weapons (OPCW) are carrying out tests on samples taken from Salisbury at the Ministry of Defence's military research facility at Porton Down, according to sources.
Mr Skripal, 66, and his 33-year-old daughter remain in a critical but stable condition in Salisbury District Hospital, Salisbury NHS Foundation Trust said.
The pair were found collapsed in the centre of the Wiltshire town two weeks ago.
A police officer who was one of the first on the scene, Sergeant Nick Bailey, has been described as conscious and in a stable condition.
The NHS trust has moved to calm concern about the dangers to the wider public.
Lorna Wilkinson, director of nursing, said: "In addition to the three inpatients at our hospital, we've been assessing and discharging a number of other people from the community who've had concerns.
"I want to reassure everyone that none of these people have needed treatment."
The investigation into what left the Skripals in a coma is now one of the biggest and most complex counter terror officers have ever undertaken, the Met Police said.
About 250 anti terrorism detectives are working on the inquiry, with many others involved, including specialists in protective equipment.
Officers are trawling through 4,000 hours of CCTV to work out Mr Skripal and Yulia's movements, and they have seized 800 exhibits and taken 400 witness statements.
A statement from the Met Police added: "The investigation is highly likely to take many months and, where it is operationally possible, updates will be issued.
"Searches are ongoing in the Salisbury area and at this stage it is not possible to put a timescale on how long these may take to conclude."
It came as specialists in protective suits and gas masks began examining a silver pick-up truck that was used to collect Yulia from the airport after she arrived from Russia on a visit to her father.
Sky sources said the car was removed from a cement works in Larkhill Road, Durrington, 10 miles north of Salisbury, around lunchtime.
The MoD confirmed it was removed in connection with the poisoning. It is thought to have been taken, along with a number of other vehicles, to Porton Down.
Ross Cassidy, a friend of the Skripals, has previously told Sky News he picked Yulia up from the airport the day before she and her father were poisoned.
A Russian government spokesman said Britain would have to provide evidence "sooner or later or apologise" for blaming it for the poisonings.
Kremlin spokesman Dmitry Peskov added that Britain's allegations were "difficult to explain…groundless and slanderous".
Foreign Secretary Boris Johnson continued his attacks on the Kremlin, claiming Vladimir Putin was using "a classic Russian strategy" of "lies and obfuscation", as he challenged Russia's denials of responsibility.
He said: "The Russian denials grow increasingly absurd.
"I think what people can see is that this is a classic Russian strategy of trying to conceal the needle of truth in a haystack of lies and obfuscation."
Theresa May also reiterated her belief that Russia was responsible, saying: "Russia has the capability and I believe the motive and intent and this is part of a pattern of behaviour we see from Russia across Europe."
At a meeting of EU foreign ministers, the bloc released a statement saying it offered "unqualified solidarity with the UK and its support, including for the UK's efforts to bring those responsible for this crime to justice".
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It called on Russia to "address urgently the questions raised by the UK and the international community and to provide immediate, full and complete disclosure of its novichok programme to the OPCW".
NATO Secretary General Jens Stoltenberg, after meeting Mr Johnson, said the first use of a nerve agent on alliance territory was a "total disregard for human life" and an unacceptable breach of international laws and rules.