Sergei Skripal and his daughter Yulia are still in critical condition as British police sieve through more than 200 witnesses and 240 pieces of evidence.
British police have identified more than 200 witnesses and are looking at more than 240 pieces of evidence in their investigation into a nerve agent attack on a Russian ex-spy and his daughter.
Former double agent Sergei Skripal, 66, and his daughter Yulia, 33, have been in hospital in a critical condition since Sunday, when they were found unconscious on a bench in the southern English cathedral city of Salisbury.
"The two victims remain in hospital and they're critical but stable," interior minister Amber Rudd told reporters on Saturday after chairing a meeting of the government's Cobra security committee.
Skripal betrayed dozens of Russian agents to British intelligence before his arrest in Moscow in 2004. He was sentenced to 13 years in prison in 2006, and in 2010 was given refuge in Britain after being exchanged for Russian spies.
Many in British media and politics have speculated that Russia could have played a part in the attack on Skripal but Rudd reiterated that it was too early to say who was responsible, and police should be given the time and space to determine the facts.
The Kremlin has denied any involvement in the incident and accused the British media of whipping up anti-Russian hysteria.
In Salisbury, normally a quiet city, military vehicles and troops in protective suits and gas masks were seen working at several of the sites associated with the Skripal investigation.
At an ambulance station a short distance from the city centre, troops in light grey overalls, purple gloves and gas masks covered ambulances with black tarpaulins as they prepared to remove them.
At the hospital where the Skripals were being treated another team used an army truck to remove a police car.
Rudd said that Nick Bailey, a police officer who became unwell after taking part in the early response to the attack, remained seriously ill but was talking and engaging with his family.
Wiltshire Police, the local force, issued a statement on Bailey's behalf in which he thanked the public for messages of support he had received.
"He does not consider himself a 'hero', he states he was merely doing his job," the statement said.
Rudd said more than 250 counter-terrorism police, from eight out of Britain's 11 specialist units, were involved in the investigation, which was proceeding with "speed and professionalism".
The military have been providing support since Friday and Rudd said the police would continue to be given all the resources they needed to conduct the investigation.
There was also a flurry of activity at the cemetery in Salisbury where Skripal's wife and son are buried, with forensic teams active in several parts of the site.
Skripal's son, Alexander, died in July last year at the age of 43. British media reported that he died while in St Petersburg, Russia. Skripal's wife, Liudmila, died of cancer at 60, in 2012.
British police have previously said they knew which exact nerve agent was used against Skripal and his daughter, but they have not made that information public.