A consortium of internet researchers claims to have identified a third Russian military intelligence (GRU) officer involved in the nerve agent attack on Sergei Skripal and his daughter Yulia in Salisbury last year.

According to the Bellingcat website, Denis Sergeev, 45, travelled on a 2 March Aeroflot flight into Heathrow, just hours before the arrival of “Alexander Petrov” and “Ruslan Boshirov”, men later identified as GRU agents Alexander Mishkin and Anatoly Chepiga – and suspects in the poisoning.

The Bellingcat team says they have determined key aspects of Mr Sergeev’s background despite a “government-level” attempt to erase records in relation to all three men.

Like both Chepiga and Boshirov before him, Denis Sergeev came from the Soviet backwaters – a small militarised town called Usharal in present-day Kazakhstan. He served in the army, in Novorossiysk on the Russian Black Sea coast, before being transferred to Moscow. There, he enrolled in the “Conservatory”, an elite military academy feeding into the GRU.

Later, he is tracked as a managing director of several sham companies, and linked to a highly unusual Russian bank loan of more than $1m.

Skripal attack aftermath – in pictures Show all 15 1 /15 Skripal attack aftermath – in pictures Skripal attack aftermath – in pictures Scene of attack Members of the emergency services in hazard suits fix the tent over the bench where Sergei and Yulia Skripal were found unconscious on a park bench in Salisbury in March 2018. Getty Skripal attack aftermath – in pictures Victim - Sergei Skripal The retired Russian colonel and former double agent for MI6 was in a critical condition in hospital for more than two months after being exposed to novichok in Salisbury. He was given refuge in the UK after being jailed in Moscow for treason. Mr Skripal came to Britain as part of a high-profile “spy swap” in 2010 in which four men were exchanged for ten Russian "sleeper agents" in the US. In this image he is speaking to his lawyer from behind bars in Moscow in 2006. AP Skripal attack aftermath – in pictures Victim - Yulia Skripal Yulia Skripal was struck down by a novichok poison alongside her father Sergei. Facebook Skripal attack aftermath – in pictures Scene of attack A police officer stands guard outside a branch of the Italian chain restaurant Zizzi where the pair dined at before falling ill. It was boarded off whilst investigators worked on the building and later found traces of the chemical weapon within it. AFP/Getty Skripal attack aftermath – in pictures Scene of attack Large areas of central Salisbury were cordoned off by police following the discovery of the Skripals. Traces of nerve agent were also found in The Mill pub. PA Skripal attack aftermath – in pictures Victim - Nick Bailey Detective Sergeant Nick Bailey, rushed to the aid of the Russian ex-spy and his daughter who were targeted with a nerve agent. He was hospitalized after aiding them and didn't leave until three weeks after the attack. Wiltshire Police/Rex Skripal attack aftermath – in pictures Police investigation - Skripal’s home Police believe they were poisoned at home, and detectives found the highest concentration of novichok on the front door of Mr Skripal’s house. Getty Skripal attack aftermath – in pictures Theresa May visits scene of attack Britain's Prime Minister Theresa May spokes with Wiltshire Police's Chief Constable Kier Pritchard near where the Skripal's were found. Britain expelled 23 Russian diplomats over the nerve agent poisoning and suspended high-level contacts, including for the World Cup on March 14. Theresa May told parliament that Russia had failed to respond to her demand for an explanation on how a Soviet-designed chemical, Novichok, was used in Salisbury. AFP/Getty Skripal attack aftermath – in pictures Skripal days before attack Sergei Skripal days before he was exposed to Novichok, that has left him fighting for life. ITV News Skripal attack aftermath – in pictures Police investigation - military involvement British soldiers were deployed soon after the attack to help a counter-terrorism investigation into the nerve agent attack. One of the places they were asked to help out with was Skripal's home and it's surrounding. They were asked to remove a vehicle connected to the agent attack in Salisbury, from a residential street in Gillingham. AFP/Getty Skripal attack aftermath – in pictures Police investigation Personnel in protective coveralls and breathing equipment cover an ambulance with a tarpaulin at the Salisbury District Hospital. AFP/Getty Skripal attack aftermath – in pictures Police investigation The investigation extended to the grave of Sergei Skripal's son Alexander in London Road cemetery. Getty Skripal attack aftermath – in pictures Police investigation The Counter Terrorism Policing Network requested assistance from the military to remove a number of vehicles and objects from Salisbury. EPA Skripal attack aftermath – in pictures Home Secretary visits scene of attack Home Secretary Amber Rudd visited the scene of the nerve agent attack at the Maltings shopping centre on 9 March. Getty Skripal attack aftermath – in pictures Yulia Skripal speaks for the first time Yulia Skripal, speaking for the first time, said she felt lucky to have survived the nerve agent attack in Salisbury which left her fighting for life. Ms Skripal said her life had been “turned upside down” by the assassination attempt. But the Russian national added she hoped to return to her homeland one day, despite the Kremlin being blamed for the attack. Reuters

Bellingcat claims travel records connect Sergeev’s cover identity, “Sergey Fedotov”, with the mysterious 2015 poisoning of a Bulgarian arms manufacturer called Emilian Gebrev. Sergeev/Fedotov’s movements in and out of Bulgaria correspond to the businessman’s initial major illness, and subsequent relapse in symptoms, the website says.

At the time of Mr Gebrev’s illness, there was much media speculation that his company’s substantial arms shipment to Ukraine might have caused the problems. His doctors, too, suspected poisoning, and sent urine samples to Verifin, a testing laboratory in Helsinki.

While the laboratory was unable to identify the substance used, it reportedly isolated traces of two organophosphates. This is consistent with the use of a nerve agent like novichok, but not exclusively so: industrial insecticides would also produce the same metabolites, for example.

On Friday, The Guardian reported that Bulgaria and the UK are jointly investigating the links between the novichok poisoning on Mr Skripal and the poisoning of Mr Gebrev.

The involvement of a “third man” in the Salisbury operation was first reported last year. In October, Russian media identified the man’s “Sergey Fedotov” cover identity. But Bellingcat’s investigation goes much further, and is the first time the man’s supposed real-life identity has been made public.

What it does not do is substantiate earlier reports that claimed Sergeev/Fedotov was responsible for reconnaissance ahead of the Salisbury attack. In Bellingcat’s estimation, the last time Sergeev was in Britain was early 2017. It would be highly unusual for that work to take place a full year ahead of the operation.

Sergeev/Fedotov's exact role remains uncertain, agrees Phillip Ingram, a former British army intelligence officer.

“Given the evidence of big money transactions, it may be that the alleged agent acted as a fixer for the Salisbury operation,” he told The Independent. “But there’s clearly another team out there apart from him. For a start, they wouldn’t have been able to do the operation without a so-called pattern of life study, which requires watching the suspect over a long time to avoid major mistakes.”

Mr Skripal, a double agent, was found unconscious on a bench together with his daughter Yulia in Salisbury in March 2018. They both survived the attack, but spent weeks in hospital, and have been moved to an undisclosed location.

The Kremlin has repeatedly denied any involvement in the poisoning, accusing British intelligence agencies of staging the attack to stoke anti-Russian hysteria. But those denials have sometimes pushed the realms credibility in the face of an increasing body of evidence produced by internet researchers like Bellingcat, and its founder Eliot Higgins.

Bellingcat’s most recent work has taken advantage of open source data and widely-available databases to highlight intelligence lapses and – quite unexpectedly – compromise the identities of possibly hundreds of GRU agents.

“If I was Eliot Higgins, I’d be very busy renewing my personal safety,” says Ingram. “These guys have a grudge, and like the rat in a corner, being poked with a stick, they will fight back.”