Police believe they have identified the suspected perpetrators of the Novichok nerve agent attack on Sergei Skripal and his daughter Yulia.
Officers think several Russians were involved in the attempted murder of the former Kremlin double agent in Salisbury back in March.
It is understood that the suspected assassins have been recorded on CCTV, which has then been cross-checked with records of people entering the UK at that time.
It comes as an inquest opens today into the death of Dawn Sturgess, 44, who died this month after apparently coming into contact with the same batch of Novichok.
It is believed that she found a bottle of the nerve agent after it had been disposed of by the attackers three months before.
Police believe they have identified the suspected perpetrators of the attack on Russian former spy Sergei Skripal and his daughter Yulia (pictured in Salisbury before the were hospitalised)
It comes as an inquest opens today into the death of Dawn Sturgess (pictured), who died this month after apparently coming into contact with the same batch of Novichok
Wednesday saw officers begin a painstaking search of Queen Elizabeth Gardens in Salisbury, Wiltshire, where they believe the couple came across the Novichok in a perfume bottle
Her partner Charlie Rowley, 45, was left fighting for his life after also being contaminated by the chemical weapon.
It is understood mother-of-three Ms Sturgess was exposed to at least 10 times the amount of nerve agent the Skripals came into contact with.
Investigators are working to the theory that the substance was in a discarded perfume bottle found by the couple in a park or somewhere in Salisbury city centre.
Mr Rowley says that after finding the bottle, Ms Sturgess sprayed Novichok straight on to her both of her wrists, the source said.
The Metropolitan Police, who are leading the investigation, declined to comment.
But a 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.'
to reports from US security officials, police have recovered grainy
CCTV images of persons who inadvertently killed Dawn.
Police outside Mr Rowley's home in Amesbury, Wiltshire, on July 5 after he and Ms Sturgess picked up a perfume bottle containing the chemical weapon Novichok
A police officer stands guard over a cordoned off rubbish bin after Dawn Sturgess and Charlie Rowley fell ill in Amesbury
Counter-terror cops are closing in on identifying the suspects, thought to be from Mr Skripal's former employers, Russia's military intelligence service the GRU.
Both he and his daughter, as well as Detective Sergeant Nick Bailey, of Wiltshire Police, made miraculous recoveries after being on the brink of death.
All three were treated at Salisbury General Hospital, where Charlie is now being cared for.
The perfume bottle could support the theory that a woman was involved in the initial hit by up to six people against Sergei and Yulia Skripal in Salisbury on March 4.
inquest will be opened today in Salisbury and the hearing is expected to
be adjourned to allow police inquiries to continue.
On Wednesday, a fingertip search of Queen Elizabeth Gardens in Salisbury was carried out.
Moscow strenuously denies poisoning Yulia (pictured) and her ex-spy father Sergei Skripal four months ago
Forensic police officers wearing hazmat suits prepare to examine a vehicle believed to belong to Sergei Skripal on March 8
Police stand guard outside the Salisbury home of Sergei Skripal, who nearly lost his life despite only coming into contact with a small amount of nerve agent, police believe
The park and other locations in Salisbury and nearby Amesbury were cordoned off last month after the exposure of the couple to the nerve agent.
Searches of properties could last months after 400 items were recovered, officers warned, while waste and litter will be removed as part of the sweep of public areas.
Last week counter-terrorism detectives revealed they had found a small bottle containing Novichok at Mr Rowley's home in Muggleton Road, Amesbury.
They are trying to establish where the container came from, and how it came to be in his house.
A team of international experts from the Organisation for the Prohibition of Chemical Weapons (OPCW) was called in to independently verify this.
They have finished collecting samples which will now be analysed at two OPCW labs before the results are reported back to the UK.
Health England said the risk to the public remains low but it continued
to 'strongly advise' not to pick up any unknown 'strange items' such as
syringes, needles, cosmetics or similar objects made of materials such
as metal, plastic or glass.
A timeline of the key developments in the Salisbury poisoning case
2010 - Sergei Skripal, a former Russian military intelligence officer jailed for spying for Britain, is released and flown to the UK as part of a swap with Russian agents caught in the United States. He settles in Salisbury.
March 3, 2018 - Yulia Skripal arrives at Heathrow Airport from Russia to visit her father in England.
March 4, 9.15am - Sergei Skripal's burgundy BMW is seen in suburban Salisbury, near a cemetery, where his wife and son are commemorated.
March 4, 1.30pm - The BMW is seen driving toward central Salisbury.
March 4, 1.40pm - The BMW is parked at a lot in central Salisbury.
A police officer stands guard outside the Zizzi restaurant where Sergei and Yulia had lunch before they collapsed in a nearby park
March 4, afternoon - Sergei and Yulia Skripal visit the Bishops Mill pub.
March 4, 2.20pm to 3.35pm - Sergei and Yulia Skripal have lunch at the Zizzi restaurant.
March 4, 4.15pm - Emergency services are called by a passer-by concerned about a man and a woman in Salisbury city centre.
Officers find the Skripals unconscious on a bench. They are taken to Salisbury District Hospital, where they remain in critical condition.
March 5, morning - Police say two people in Salisbury are being treated for suspected exposure to an unknown substance.
Detective Sergeant Nick Bailey was among the first police officers on the scene and was himself hospitalised
March 5, afternoon - Wiltshire Police, along with Public Health England, declare
a 'major incident'
March 7 - Police announce that the Skripals were likely poisoned with a nerve agent in a targeted murder attempt.
They disclose that a police officer who responded to the incident is in serious condition in a hospital.
March 8 - Home Secretary Amber Rudd describes the use of a nerve agent on
UK soil was a 'brazen and reckless act' of attempted murder
March 9 - About 180 troops trained in chemical warfare and decontamination
are deployed to Salisbury to help with the police investigation.
Russian Foreign Minister Sergey Lavrov says Moscow might be willing to assist with the investigation but expresses resentment at suggestions the Kremlin
was behind the attack.
March 11 - Public health officials tell people who visited the Zizzi restaurant or
Bishops Mill pub in Salisbury on the day of the attack or the next day to wash their clothes as a precaution.
March 12, morning- Prime Minister Theresa May tells the House of Commons that the Skripals were poisoned with Novichok, a military-grade nerve agent developed by the Soviet Union during the Cold War.
March 12, afternoon - Public Health England ask everyone who visited Salisbury town centre on the day of the attack to wash all of their clothes and belongings.
Officers wearing chemical protection suits secure the forensic tent over the bench where Sergei and Yulia fell ill
March 14 - The PM announces the expulsion of 23 suspected Russian spies from the country's UK Embassy.
March 22 - Nick Bailey, the police officer injured in the attack, is released from hospital.
March 26 - The United States and 22 other countries join Britain in expelling scores of Russian spies from capitals across the globe.
March 29 - Doctors say Yulia Skripal is 'improving rapidly' in hospital.
April 3 - The chief of the Porton Down defence laboratory said it could not verify the 'precise source' of the nerve agent.
April 5, morning - Yulia Skripal's cousin Viktoria says she has received a call from Yulia saying she plans to leave hospital soon.
Dawn Sturgess died in hospital on July 8
April 5, afternoon - A statement on behalf of Yulia is released by Metropolitan Police, in which she says her strength is 'growing daily' and that 'daddy is fine'.
April 9 - Ms Skripal is released from hospital and moved to a secure location.
May 18 - Sergei Skripal is released from hospital 11 weeks after he was
June 30 - Dawn Sturgess and Charlie Rowley fall ill at a property in Amesbury, which is eight miles from Salisbury, and are rushed to hospital.
July 4 - Police declare a major incident after Ms Sturgess and Mr Rowley are exposed to an 'unknown substance', later revealed to be Novichok.
July 5 - Sajid Javid demands an explanation over the two poisonings as he
accuses the Russian state of using Britain as a 'dumping ground for poison'.
July 8 - Mother-of-three Dawn Sturgess, 44, dies in hospital due to coming
into contact with Novichok.
July 10 - Mr Rowley regains consciousness at hospital, and later tells his brother that Dawn had sprayed the Novichok onto her wrists.
July 19 - Police are believed to have identified the perpetrators of the attack.