British intelligence services investigated "increased" and "unusual" activity at the Russian embassy in London in the days before and after the Novichok poisoning, the Press Association understands.
MI5, MI6 and GCHQ looked into "frantic comings and goings" at the building in Kensington in the days leading up to the moment former Russian spy Sergei Skripal and his daughter Yulia came into contact with the nerve agent and after they were found unconscious, according to a source.
In the wake of the attack, which took place in Salisbury a year ago on Monday, the observations were "deemed to be significant and of interest", it is understood.
A security source told the Press Association: "The intelligence agencies have been investigating unusual and increased activity at the Russian embassy in Kensington in the days leading up to and after the attack on the Skripals."
The source, who has knowledge of the investigation, said lines of inquiry included looking at movements - described as "frantic and unprecedented" - which were observed at the time, adding: "As would be expected, the UK security services have eyes on known and undeclared foreign intelligence operatives."
Britain has accused Russia of being behind the attack.
Intelligence services continue to believe it was ordered at "the highest level in the Russian state", the source said.
Some two weeks after the attack, British Prime Minister Theresa May kicked out 23 suspected Russian spies from the London embassy in the largest mass expulsion of diplomats since the Cold War.
Both Mrs May and British Home Secretary Sajid Javid claimed the Russian government had been involved, saying approval came from a senior level in Moscow.
It is understood several Russians are being investigated over the attack.
The source did not rule out the possibility of there being more suspects than those already named and said inquiries were "not limited to the suspects already publicly declared."
In September, Scotland Yard and the UK's Crown Prosecution Service said there was sufficient evidence to charge two Russians - known by their aliases Alexander Petrov and Ruslan Boshirov - with offences including conspiracy to murder after they were caught on CCTV in Salisbury the day before the attack.
They are accused of being members of the Russian military intelligence service the GRU.
In February, high-ranking Russian military intelligence service officer Denis Sergeev was revealed as being reportedly in Britain during the attack.
The 45-year-old member of the GRU, who uses the alias Sergei Fedotov, arrived 48 hours before the poisoning but it is unclear what role, if any, he played in the attack, the investigative website Bellingcat said.
Detectives believe the Skripals first came into contact with the nerve agent when it was sprayed on the door handle of their home in Christie Miller Road, Salisbury.
Mr Skripal, 66, and his 33-year-old daughter survived the attack, as did Wiltshire Police Detective Sergeant Nick Bailey, who came into contact with the poison when he searched their home.
But Dawn Sturgess, 44, fell ill in Amesbury months after the incident and died in hospital in July after coming into contact with a perfume bottle believed to have been used in the attack on the Skripals and then discarded.
The specially adapted counterfeit Nina Ricci bottle - which Ms Sturgess handled - is thought to have contained the substance.
Her partner Charlie Rowley was also exposed to the same nerve agent but was treated and discharged.
The 45-year-old demanded justice on Friday as the city was declared "safe" from Novichok after the decontamination process was completed.
Mr Rowley said the case was still an "open book" after counter-terrorism police re-issued an appeal for information about the perfume bottle, admitting they still do not know what happened to it between March 4 and when he said he found it on June 27.
On Sunday, the 20-year-old son of Ms Sturgess, Ewan Hope, urged Russian President Vladimir Putin to hand over the men suspected of being responsible for her death.
His grandparents previously called on the UK Government for answers.
Russia has repeatedly denied any involvement with Mr Putin claiming the two suspects were civilians.
During an interview the pair said they were tourists visiting Salisbury - particularly its famous cathedral.
A fortnight after the attack, the Russian embassy in the UK's official Twitter account posted a comment which said: "In absence of evidence, we definitely need Poirot in Salisbury!" with an accompanying picture of actor David Suchet in costume as his famed character.
A week later, Russia's ambassador in London was ridiculed for claiming Britain was behind the attack.
Following a press conference at the embassy, Alexander Yakovenko was said to have been called a Russian "Comical Ali" by Conservative MPs, after Saddam Hussein's spokesman Mohammed Saeed al-Sahhaf - who earned the nickname over his claims.
The UK's Home Office, the Foreign and Commonwealth Office, and GCHQ all declined to comment.
The Russian Embassy did not respond when contacted.