The Salisbury spy probe could take several weeks as investigators carry out a "painstaking" operation to identify how a nerve agent was used to poison a former double agent on British soil.
Giving the first investigative update on the case since Theresa May publicly pointed the finger at Russia, counter-terror police chief Neil Basu said officers' "prime focus" is to establish the method used to administer the chemical weapon.
A huge police inquiry was launched after former double agent Sergei Skripal, 66, and his daughter Yulia, 33, were found slumped on a bench in Salisbury, Wiltshire, on March 4. They remain in a critical condition.
Detective Sergeant Nick Bailey, who was part of the initial police response, was also taken ill. He is in a serious but stable condition.
Speaking at Scotland Yard, Mr Basu said: "The public are going to continue to see a great deal of police activity in and around the city, including potentially more cordons being erected, but please don't be alarmed.
"It is necessary as part of this major investigation by the counter-terrorism policing network. In truth it may last many weeks."
The Metropolitan Police Assistant Commissioner would not identify any potential suspect, saying: "It's a painstaking operation to identify anyone of interest to this inquiry, and eliminate them or include them, but at this stage we are not declaring a person of interest or a suspect."
Investigators are also focusing on Mr Skripal's red BMW, registration number HD09 WAO, and appealing for any witnesses who saw the pair in the car between 1pm and 1.45pm on March 4 to come forward.
Police have so far collected 380 exhibits and have been scouring hours of CCTV footage from across the city.
Detailing the timeline leading up to the pair being taken ill, Mr Basu disclosed that Yulia had arrived at Heathrow Airport on a flight from Russia the day before.
The senior officer referred to Mr Skripal as a British national and Yulia as a Russian citizen.
He also revealed that a total of 38 people had been seen by medics in relation to the incident, of whom 34 had been assessed and discharged from hospital.
Three people remain in hospital - the Skripals and Mr Bailey. One other unnamed person is being monitored as an outpatient, but is not showing signs of exposure to the chemical weapon.
As the clock ticked towards the midnight deadline Theresa May set for Moscow to explain how a military-grade chemical from a group of nerve agents known as Novichok came to be used on British soil:
- It emerged that police and MI5 are to look into allegations that a string of other deaths on UK soil may be linked to Russia.
- Peter Wilson, the UK's permanent representative to the Organisation for the Prohibition of Chemical Weapons, said the "indiscriminate and reckless" attack on the Skripals was the first offensive use of a nerve agent of any sort on European territory since the Second World War.
- The Government was weighing up Britain's options should Russia not provide a satisfactory response - with a cyber counter-strike said to be among the possible measures being considered, along with economic, financial and diplomatic action.
- Russian foreign minister Sergei Lavrov insisted the country was not to blame, and asked for access to samples of the poison.
- France, Germany and former US secretary of state Rex Tillerson, speaking before he was sacked, all gave their backing to the UK.
- Ahead of a call with Mrs May, US president Donald Trump said: "It sounds to me like it would be Russia based on all of the evidence they have."
The episode has left Britain's relations with Moscow, which were already under severe strain, at breaking point.
On Monday, Mrs May said the Government had concluded it is "highly likely" Russia was responsible.
She demanded that Moscow account for how a Russian-produced nerve agent could have been deployed in Salisbury and vowed to set out a "full range" of measures Britain will adopt if no credible response is received by the end of Tuesday.