Police are mounting a massive investigation to establish how former Russian double agent Sergei Skripal and his daughter were targeted with a nerve agent and who was responsible.
Hundreds of officers are working round the clock on the probe - one of the most politically sensitive for years.
Investigators are trawling CCTV as they attempt to piece together a detailed timeline of events leading up to the discovery of Mr Skripal, 66, and his daughter Yulia, 33, on a bench in Salisbury, Wiltshire, on Sunday.
Police have declined to specify the nerve agent used, which was identified in scientific tests by Government experts.
Establishing the substance's origins will be an urgent priority as authorities attempt to track down the person or people responsible.
Metropolitan Police Assistant Commissioner Mark Rowley said: "Our role now is to establish who is behind this and why they carried out this act.
"To that end we have hundreds of detectives, forensic specialists, analysts and intelligence officers working together around the clock on the case, for example examining hundreds of hours of CCTV and building a detailed timeline of events."
He also re-iterated an appeal for anyone with information about the incident to come forward, including those who visited the area near where the pair were taken ill on Sunday.
Although the alert has not been declared as a terrorist incident, the investigation is being led by the national counter-terrorism police network.
While the network's main role is to prevent and investigate terrorist activity, it can also be assigned to other specialist probes including inquiries relating to war crimes, counter-espionage and the Official Secrets Act.