The investigation into Fedotov is hugely sensitive and Scotland Yard has declined to comment on it.Police have released CCTV footage showing Chepiga and Mishkin in Salisbury on the day of the attack and the day before it. It is not clear if footage exists of Fedotov in the city at the same time although there will be images of him coming into the country and leaving via Heathrow airport.Col Skripal was a senior officer in the GRU caught selling secrets to MI6. He moved to the UK in 2010 in a spy swap. Counter-terrorism police are investigating a third suspect in the Salisbury nerve agent attack amid suggestions he acted as look out for two Russian military intelligence assassins.Investigators have identified a “third man” in the poisoning of Colonel Sergei Skripal as a Russian national travelling under the name Sergei Fedotov.Flight details obtained by an independent and respected Russian website show Fedotov arriving in the UK on March 2 on a different flight but on the same day as two hitmen – now named as Anatoliy Chepiga and Alexander Mishkin, both revealed as highly decorated and senior members of the GRU, Russia’s military intelligence unit. Fedotov then left the UK on March 4 on the same flight as Chepiga and Mishkin.The Telegraph understands that the Metropolitan Police, which is investigating the nerve agent attack in the Wiltshire city, is reviewing evidence with a view to considering whether or not to bring charges against Fedotov. Chepiga and Mishkin – under their aliases Ruslan Boshirov and Alexander Petrov – have already been charged in absentia with the attack.One source suggested that Fedotov may have been in the UK to oversee the operation. It is thought he may have travelled to Salisbury as look out on the day of the assassination attempt, making sure Col Skripal and his daughter Yulia were out of the house before Chepiga and Mishkin, a military doctor, smeared his front door handle with novichok nerve agent. 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.