The investigation believes that potential assassins used a perfume bottle with Novichok to spray the nerve agent on the doorknob of Skripal's house in March.

Detectives believe that the Salisbury nerve agent attack was probably carried out by present or former agents of Russia’s military intelligence service, as reports emerged that a number of suspects have been identified.

The Times understands that links to the GRU are being examined as part of the investigation into the botched attempt to kill Sergei Skripal, the former Russian double agent, and his daughter. Other possibilities, such as the involvement of another Russian spy agency, have not been ruled out.

Skripal, 67, was an officer in the GRU when he was recruited by MI6 in the 1990s. The Russian intelligence officers he identified were almost all former soldiers. Of all the branches of Russian intelligence, the GRU adheres most strongly to a stern loyalty code.

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The investigation believes that potential assassins used a perfume bottle with Novichok to spray the nerve agent on the doorknob of Skripal's house in March. Then the perpetrators threw the bottle away before fleeing the country.

Four months later, Dawn Sturgess, 44, and her partner Charlie Rowley found that very bottle. Sturgess died on July 8 from Novichok poisoning while Rowley is still being treated at a hospital.

It is believed that Sturgess sprayed what she thought was perfume on her body and was thus affected by a dose 10 times larger than that used against the Skripals.