Hundreds of officers are continuing to work around the clock gathering evidence to identify those responsible for the attempted murders in Salisbury town centre on Sunday, 4 March.
As widely reported, Sergei Skripal, a 66-year-old British national, and his 33-year-old daughter Yulia Skripal, a Russian citizen, remain in a critical condition in hospital after being exposed to a nerve agent in a targeted attack.
Wiltshire Police officer Detective Sergeant Nick Bailey, who was amongst those first to respond to the incident, was also taken ill and remains in a serious but stable condition in hospital.
Met Assistant Commissioner Neil Basu said: "This is an extremely challenging investigation. You will understand, the police and its partners have been dealing with a number of unique and complex issues.
"Our thoughts are with the victims and their families at this very difficult time.
"Hundreds of officers, including local police teams, detectives from across the counter-terrorism network, forensic experts, intelligence analysts and the military are combing the area for evidence to establish who is responsible for this reckless and despicable act.
"As the Prime Minister said yesterday we are sifting and assessing all evidence available as we explore all investigative avenues. This includes extensive CCTV footage from across the city and over 380 exhibits so far.
"The public will continue to see a lot of police activity in and around the city, including new cordons being established, while existing cordons may remain in place for some time. The number of scenes and their locations is likely to change over the coming days. This is nothing for the public to be alarmed about, but is necessary as part of this major investigation by the counter-terrorism policing network.
"We are prioritising different scenes at different times, depending on where the evidence takes us and the latest scientific advice, while managing the welfare of those attending the scenes."
Public safety continues to be the priority of all agencies working together as part of this national response and we continue to liaise closely with public health and science experts.
We do have a good understanding of the nerve agent, which has enabled the authorities to assess and mitigate the health risks around it.
Anyone with concerns about their health should contact their GP; to ring the NHS helpline on 111 or call 999, depending on the severity of the symptoms.
AC Basu said: "I accept there is huge public interest in this incident. However with such a sensitive and complex inquiry, you will understand that I can’t say anything that may impact on the important work being undertaken by our very best officers in counter-terrorism policing.
"I would like to praise and thank the first responders for their immense bravery, as well as the officers and staff who continue to work tirelessly on this investigation. They are supported by hundreds of staff from the ambulance service, fire service and military who are doing an incredible job. I thank them too.
"I’d also like to thank the public for the fantastic support they have shown so far. We’ve had a great response to our appeals and continue to encourage people to come forward.
"In particular, we are appealing for anyone who saw the Skripals or their car, a red BMW with the registration plate HD09 WAO, between approximately 13:00hrs and 13:45hrs on Sunday, 4 March in the Salisbury area to contact police.
"An investigation of this complexity and size will understandably take some time and we ask for the public’s continued support and patience for what we know is a difficult time for them also."
Our work to understand the events leading up to Sergei and Yulia becoming seriously ill is ongoing but at this time we know the following:
Yulia arrived at Heathrow Airport on a flight from Russia at approximately 14:40hrs on Saturday, 3 March.
At approximately 13:40hrs on Sunday, 4 March, Sergei and Yulia arrived in Sainsbury’s upper level car park in the Maltings.
At some time after this, they went to the Bishops Mill Pub before going to Zizzi Restaurant at approximately 14.20hrs, where they stayed until around 15:35hrs.
Emergency services received a report from a member of the public at 16:15hrs and police officers arrived within minutes at the scene to find Sergei and Yulia extremely ill on a park bench near the restaurant. Subsequently, scientists have determined this to be the result of an identified military grade nerve agent.
We have received lots of calls from the public and continue to urge anyone who was in Salisbury between 13:00hrs and 16:00hrs on Sunday, 4 March to call police on 101. In emergencies always call 999.
Any members of the public who have images or footage that may assist the investigation should send them to this address www.ukpoliceimageappeal.co.uk
Wiltshire Chief Constable Kier Pritchard: "As you would expect, Wiltshire Police is being fully briefed and supporting the ongoing investigation. This includes officers and PCSOs being dispatched to and managing cordons at scenes.
"We will continue to provide a high visible policing presence in the city to try to allay public concern. Any member of the public with concerns regarding scenes should speak to any of my officers or PCSOs at the locations.
"We thank the public for their continued support."