Show more Police are still investigating when Chepiga and Mishkin were given the Novichok nerve agent and how they disposed of it after the attack in March.The Telegraph first reported last September that British investigators believed that a third GRU military intelligence officer had travelled to the UK to reconnoiter Salisbury before the hit on former Russian agent Mr Skripal. Sergeyev is believed to have been born on Sept 17 1973 in a military town in what is now Kazakhstan.An investigation by Bellingcat, Russian site The Insider and Czech outlet Respekt concluded that he served in the army in southern Russia before studying at a military diplomatic academy in Moscow, the “GRU conservatory” that trains 100 officers each year. The Telegraph uncovered additional information confirming Sergeyev’s GRU ties and found that the Russian authorities appear to have already made attempts to scrub his name from publicly available information.In 2010, Sergeyev received his fake “Fedotov” passport at the same Moscow desk that gave Chepiga and Mishkin their cover identity documents.The spy was known to have travelled extensively under the guise of Fedotov, and possibly even stayed in the UK after the attempt to poison Mr Skripal. He took 10 calls from the Russian number while in London – and phoned it himself before departing Paddington on March 4, the day Mr Skripal and his daughter Yulia were found unconscious.Theresa May called for an end to Russia’s “irresponsible and destabilising activity”, describing the Salisbury poisoning as a “truly despicable act” before the developments emerged on Friday. As well as the Skripals, DS Nick Bailey and Charlie Rowley all fell ill after being contaminated by Novichok.Mr Rowley’s partner Dawn Sturgess, a mother-of-three, died in July last year after she came into contact with the nerve agent.Russia denies any involvement in the attack. Denis Vyacheslavovich Sergeyev is said to be a high-ranking GRU officer and a graduate of Russia’s Military Diplomatic Academy. The “third man” involved in the Salisbury poisoning commanded the Russian military team which carried out the attack, phone records suggest. Denis Sergeyev, who the Telegraph first revealed was suspected in the operation, is believed to hold the rank of major general in the GRU.He travelled to London under the alias Sergei Fedotov, arriving the same day as Anatoly Chepiga and Alexander Mishkin, the suspected GRU agents who smeared the Novichok nerve agent on Sergei Skripal’s door handle in March.Evidence uncovered by the investigative website Bellingcat suggests that Sergeyev liaised with superior officers in Moscow while he was in the UK.Sources speaking to BBC’s Newsnight have pointed to him as the operational commander of the attempted assassination. While travelling to the UK during 2017-2018, Sergeyev only spoke to a single telephone number, a Russian “ghost mobile” believed to be his connection to GRU headquarters, phone data suggests. Want the best of The Telegraph direct to your email and WhatsApp? Sign up to our free twice-daily Front Page newsletter and new audio briefings.