Russia angrily denied the accusation as hysterical Western propaganda, but the poisoning has escalated into an international crisis that has isolated the Kremlin. Two dozen countries, including the United States, sided with Britain, ordering scores of Russian diplomats expelled in retaliation.
Until now, investigators had been exploring the different ways the poison could have been delivered: a bouquet of flowers, or granules of the nerve agent in the ventilation system of Mr. Skripal’s car.
Trace elements of the nerve agent had been found in several spots, including a table where they had eaten lunch. Detectives pored over 5,000 hours of CCTV footage taken in Salisbury, looking for people who might have sprayed it on them, and taken statements from 500 witnesses.
Sgt. Nick Bailey, an emergency medical worker who was exposed to the nerve agent, became ill after going to the Skripal house, suggesting that the exposure occurred there.
The police will now remove cordons from a number of spaces the Skripals visited on the day they were sickened, including a mall and the cemetery where their family members, Lyudmila and Aleksandr Skripal, are buried.
The Skripals remain hospitalized, in critical condition. A court order allowing investigators to take blood samples for analysis last week held out little hope that either would fully recover.
“The precise effect of their exposure on their long-term health remains unclear, albeit medical tests indicate that their mental capacity might be compromised to an unknown and so far unascertained degree,” the court order read.