British counter terrorism officers are trying to establish how a rare nerve agent was used to target former Russian double agent Sergei Skripal.
Mr Skripal and his daughter fell violently ill during night out in Salisbury several days ago and remain in a critical condition in hospital. The police officer first on the scene was also seriously affected but has now emerged from a coma.
Several locations around the town remain cordoned off, following the incident which Britain's Home Secretary Amber Rudd has described as a brazen and reckless attempted murder.
The BBC's Tom Symonds in Salisbury tells Susie Ferguson the poisoning has been "effectively the use of a chemical weapon".