A third suspect linked to the poisoning of the former Russian double-agent Sergei Skripal and his daughter in Salisbury has been identified as a high-ranking officer in Russian military intelligence by the investigative website Bellingcat.

Bellingcat reported that Denis Sergeyev, a graduate of the Military Diplomatic Academy, a training ground for intelligence officers, is the real identity of a man operating under the cover persona of Sergey Fedotov.

Two other Russian military intelligence officers – who were first identified by Bellingcat – have already been charged in absentia by the UK over the March 2018 attack.

According to the website, Sergeyev was in Bulgaria in 2015 when a businessman there fell into a coma along with his son and one of his company directors. All three survived the suspected poisoning, but the case was never solved. British investigators are on the ground to investigate any link between the cases, the Bulgarian PM told the Guardian on Thursday.

British authorities have not identified a third suspect, and the Bellingcat report acknowledged that “it is unclear what [Sergeyev’s] role may have been, if any, in the preparation and execution of the poisoning operation”.

Skripal, a former GRU officer, was convicted of high treason and jailed for 13 years in Russia in 2004, after he shared information on dozens of his fellow spies with MI6. He settled in the UK in 2010 after a spy swap.

On 4 March last year he and his daughter Yulia, 34, who was visiting him from Moscow, were found unconscious in the centre of Salisbury. Officials from the UK government’s chemical weapons laboratories from Porton Down have said they were poisoned with a nerve agent called novichok, which was developed by Soviet chemists.

Two Russians, who identified themselves as Alexander Petrov and Ruslan Boshiro – the aliases used by the agents charged by the UK – appeared on Russian TV to say they were innocent tourists who had gone to Salisbury to see its cathedral.

Bellingcat claimed that Russian authorities have erased the public records of Sergeyev, Alexander Mishkin and Colonel Anatoliy Chepiga – Petrov and Boshiro’s real names – over the past two months.

The use of a nerve agent in the UK by Russian agents has led to the worst diplomatic fallout between London and Moscow since the cold war. The UK and its allies expelled more than 150 Russian diplomats as a result of the poisoning.

Neither Sergei Skripal nor his daughter have appeared in public since the poisonings, although both have reportedly been discharged from hospital. A police officer also affected by the poison, DS Nick Bailey, returned to work last month after 10 months of recuperation.

Dawn Sturgess, 44, fell ill in Amesbury months after the incident after coming into contact with a counterfeit Nina Ricci perfume bottle believed to have been used in the attack on the Skripals and then discarded. She died in hospital in July. Her partner, Charlie Rowley, 45, was also exposed to the same nerve agent but was treated and discharged.