The police investigation into the poisoning of a couple in Wiltshire from a suspected nerve agent which nearly killed a former Russian spy and his daughter is expected "to take months to complete", according to officers.
Charlie Rowley and Dawn Sturgess were found unconscious in Wiltshire on Thursday after "handling a contaminated item" believed to have traces of novichok, the same group of nerve agents which put Sergei and Yulia Skriptal in critical condition in early March.
Police have not been able to rule out links between the incident in Amesbury this week and the poisoning 12 miles away in Salisbury of Sergei Skripal, an ex-Russian military intelligence officer who acted as a double agent for the UK's intelligence services during the 1990s and early 2000s.
The Metropolitan Police said: "Due to the unique challenges involved with this operation, police activity is expected to take weeks and months to complete."
Roughly 100 counter-terror officers are working "round the clock" with Wiltshire Police to identify the source of contamination, according to the Met.
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Relations between Britain and Russia have soured since the Salisbury attack, with Theresa May saying it was "highly likely" that Moscow was behind the poisoning of Sergei Skripal, an allegation which Russia strongly denies.
Home secretary Sajid Javid said earlier this week that there was a "strong working assumption" the couple came into contact with a similar nerve agent in a different location to the clean-up sites in nearby Salisbury.