More than 200 witnesses have been identified as police investigate the Russian ex-spy poisoning “at speed”, the home secretary has said.
Amber Rudd praised the professionalism of the police who are now looking at more than 240 pieces of evidence.
The investigation into the attempted murder of Sergei Skripal and his daughter Yulia is now in its sixth day.
Meanwhile, Det Sergeant Nick Bailey, who fell ill attending the pair, has denied he is a “hero”.
Mr Bailey remains seriously ill but is awake and engaging with his family.
Ms Rudd said both Col Skripal, 66, and his daughter, 33, who are being treated at Salisbury District Hospital, remain in a “critical but stable condition” after being exposed to a nerve agent.
The pair were found unconscious on a bench in Salisbury, Wiltshire, on Sunday 4 March.
More than 250 counter terrorism police are now involved in what the home secretary has described as a “major investigation”.
Speaking after a meeting of the government’s emergency committee, Cobra, Ms Rudd said the government was using enormous resources to try and identify those responsible for the attempted murder.
She added: “This investigation is focused on making sure that we keep people safe and also that we collect all the evidence so that when it comes to attribution [of the attack] we will be absolutely clear where it should be.
“The police have said that if anybody thinks they have any additional information they would welcome them coming forward.
“There is also substantial amounts of CCTV they have to go through. This is a painstaking, detailed investigation and the police need to be given the space and time to get on with it.”
Russia has denied any involvement. UK Foreign Secretary Boris Johnson has said the country will respond “robustly” if Moscow is found to have been behind the incident.
As part of the investigation, about 180 military personnel have been deployed to help remove vehicles and objects which may have been contaminated.
They include Army personnel, Royal Marines, members of the RAF and others who are specially trained in chemical warfare and decontamination.