Police are believed to have identified the suspected perpetrators of the poisoning of a Russian ex-spy and his daughter in March, reports say.
Several Russians were thought to be involved in the attempted murder of Sergei and Yulia Skripal in Salisbury, sources told the Press Association.
They are believed to have been identified through CCTV, cross-checked with border entry data.
Earlier this month, Dawn Sturgess, 44, died after being poisoned by Novichok.
She and her partner, Charlie Rowley, 45, fell ill on 30 June in Amesbury, Wiltshire. He remains seriously ill in hospital.
Police believe the incident is linked to the poisoning of the Skripals, who were discovered slumped on a bench on 4 March. They have since been discharged from hospital.
The UK government has blamed Russia, but the country's authorities deny any involvement.
The source with knowledge of the investigation told the Press Association: "Investigators believe they have identified the suspected perpetrators of the Novichok attack through CCTV and have cross-checked this with records of people who entered the country around that time.
"They (the investigators) are sure they (the suspects) are Russian."
A source also said to have knowledge of the investigation told CNN on Thursday that two suspects had been identified and that the pair had left the UK after the attack.
The suspects have not been named.
Asked about the latest developments, the Russian ambassador to the UK Alexander Yakovenko told the BBC: "Unfortunately, we don't have official statement of the British side.
"I want to hear that from the Scotland Yard or from the Foreign Office. A lot of versions that we hear in newspapers, they are not supported by the Foreign Office."
The prime minister's deputy official spokeswoman said: "This is an ongoing complex investigation and I can't comment on the speculation around it.
"We need to make sure the police have the time and space to carry on with the investigation."
But, writing on Twitter, security minister Ben Wallace called the reports "ill informed and wild speculation".
The Met Police, who is leading the investigation, and the Home Office have both declined to comment.
The BBC has not been able to independently confirm the story.
Philip Ingram, a former British Army intelligence officer and chemical weapons expert, said the latest development supported his perception that this was a "professional attack" designed to send a "political message" - adding that the poisoning happened two weeks before the Russian election.
"My view is that the primary reason behind it was to send a message out to dissenters - and Sergei Skripal was chosen because he was based in Salisbury and that gave the Russians plausible deniability by saying, oh it must have leaked from Porton Down, because it's just up the road," he said.
Porton Down is a military research base near Salisbury.
An inquest into the death of Ms Sturgess was opened and adjourned on Thursday. Coroner David Ridley said the cause of death would not be given until further tests are completed.
The victim's sister, Stephanie Burgess, said she had visited her hours before she died.
In a statement read at the inquest, she said: "I was informed that a decision had been made that the medical staff were going to turn off or reduce oxygen and this would likely result in Dawn's death.
"I then said my goodbyes to Dawn before leaving hospital."
Counter-terrorism detectives have revealed they found a small bottle containing Novichok at Mr Rowley's home in Muggleton Road, Amesbury.
They are trying to establish where the container, thought to be a bottle of perfume, originated from, and how Mr Rowley and Ms Sturgess first encountered it.
On Wednesday, international chemical weapons experts completed their investigations in Amesbury, where they sought to identify whether the substance that poisoned the couple was from the same batch used against the Skripals.
Other operations are ongoing, with police divers seen going into a local river and officers searching a playground in Salisbury on their hands and knees.
The risk to the public remains low, according to Public Health England.
Mike Wade, deputy director for health protection in the South West, said: "The advice remains - if you didn't drop it, then don't pick it up."