Police on Friday raced to find the object that contaminated a British couple with the Soviet-made Novichok nerve agent in southwestern England where a former Russian spy was poisoned with the same toxin four months ago.
Dawn Sturgess, 44, and Charlie Rowley, 45, fell ill on Saturday in Amesbury, a small town near the city of Salisbury where Sergei Skripal and his daughter Yulia collapsed on March 4, spreading fear once again among locals.
Police said they had established that the couple, who remain in a critical condition in hospital, were exposed to the nerve agent after "handling a contaminated item."
They also did not rule out the possibility of more people coming into contact with the poison, which they suspect may have been left over from the attempted murder on the Skripals, although police have yet to determine whether it was the same batch.
"It is rather scary," local resident Geoffrey, 66, told AFP, as he walked by the canal.
"It is an agent, it is not a gun or a knife that you can find and dispose of. It is something different, it could be on that bench... it makes me worried."
Dentist Jane Turner, 57, said she was concerned about children who "can pick things up without even thinking".
London blames Russia for the Skripal attack, with interior minister Sajid Javid on Thursday accusing Moscow of using Britain as a "dumping ground for poison".
Russia has strongly denied the accusation.
"It is now time that the Russian state comes forward and explains exactly what has gone on," Javid told parliament.
"It is completely unacceptable for our people to be either deliberate or accidental targets, or for our streets, our parks, our towns to be dumping grounds for poison."
But Russia quickly hit back, denouncing Britain for playing "dirty political games", trying to "muddy the waters" and "frighten its own citizens".
"We urge British law enforcement not to get involved in dirty political games," foreign ministry spokeswoman Maria Zakharova told reporters.
"This government and its representatives will have to apologise to Russia and the international community," she said.
Kremlin spokesman Dmitry Peskov described the case as "very worrying" but said Russia had no information "about what substances were used and how they were used".
The Skripal incident triggered a major diplomatic crisis, leading to Britain and its allies withdrawing diplomatic staff from Moscow and tit-for-tat expulsions by Russia.
Novichok is a military-grade nerve agent developed by the Soviet Union during the Cold War.
A government scientist told the BBC on Thursday that the agent can be degraded by water and sunlight, meaning it was unlikely the contamination took place in the open, and said that it was so toxic that it could pass through the skin.
Police declared a major incident on Wednesday after Sturgess and then Rowley collapsed on Saturday.
They initially suspected that the couple had consumed a contaminated batch of illegal drugs, saying they had found "paraphernalia" in the house, but tests at nearby defence laboratory Porton Down revealed they had been exposed to Novichok.
A friend of Rowley's told AFP that he was a drug user and Sturgess lived in a homeless hostel in Salisbury.
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.
Several sites in the city and nearby Amesbury that were visited by the couple have been cordoned off, including a park and Sturgess's homeless hostel in Salisbury, as well as a pharmacy, church and the house in Amesbury where they fell ill.
Officials said there was only a "low risk" to the wider public, but urged anyone who had visited the affected sites to wash their clothes and wipe down personal items.
The Novichok used in March was believed to have been smeared on the Skripal's front door.
Police said there was no evidence the latest victims had visited any of the sites linked to the Skripals, which have since been decontaminated.
Sam Hobson, a friend of the couple, said he had visited Salisbury with them the day before they fell ill.
Hobson said he went to Rowley's house on Saturday as Sturgess was being taken to hospital and stayed with him for several hours until he too began to complain of feeling ill.
"He was sweating loads, dribbling, and you couldn't speak to him," Hobson said.
"It's like he was in another world, hallucinating."