Police have identified more than 240 witnesses and are examining more than 200 pieces of evidence as they investigate the attempted murder of a former Russian spy in Salisbury.
Sergei Skripal, 66, and his daughter Yulia, 33, are in a critical condition in hospital after being attacked with a nerve agent in the cathedral city on Sunday.
Detective sergeant Nick Bailey, who fell ill while trying to help the pair, is in a serious condition but has been able to talk to those visiting him in hospital.
After chairing a meeting of the Government’s Cobra emergency committee, Home Secretary Amber Rudd gave details of the investigation, which involves more than 250 counter-terror officers.
She said: “We need to give the police and the investigators the space to get on with [it].
“I want to stress that they are proceeding with speed and professionalism.
“We are putting in enormous resources to ensure that they have all the support that they need to do that.”
Ms Rudd did not give details on the nerve agent that was used and would not be drawn about who might have been responsible.
She said: “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.”
Meanwhile, DS Nick Bailey, who was one of the first on the scene after the Skripals were found, thanked the public for their support during his recovery.
A statement released by Wiltshire Police said: “Nick would like us to say on his behalf that he and his family are hugely grateful for all the messages of support from the public, and colleagues from the police family.
“People have been so kind and he has expressed that he will never forget that kindness.
“He also wishes to say that he was part of a group of officers and other emergency service colleagues who dealt with the initial incident.
“He wants to say that he does not consider himself a ‘hero’, he states he was merely doing his job – a job he loves and is immensely proud of – just like all of his other dedicated colleagues do, day in day out, in order to protect the public and keep people safe.”
Some 21 people required medical treatment in the days following the poisoning.
Earlier on Saturday, specialist military help was used to remove vehicles and objects from scenes across Salisbury amid fears of contamination.
Around 180 troops, including chemical specialists, have been deployed in the city to help with the investigation.
Officers in hazmat suits worked at a cemetery where there are memorials for both Mr Skripal’s son Alexander, who was cremated last year, and wife Liudmila, who was buried there in 2012.