Sergei Skripal and his daughter Yulia were poisoned by a nerve agent at the former Russian spy’s home, police have now confirmed.
Detailed forensic testing revealed the highest concentration of the nerve agent was found on the front door of Mr Skripal’s home in Christie Miller Road, in Salisbury.
The Metropolitan Police said traces of the nerve agent had been found at some of the other scenes detectives have been working at, but at lower concentrations.
Detectives will now focus their inquiries in and around the address, and specialist teams will step back from some of the other areas investigated over the past few weeks.
Deputy assistant Commissioner Dean Haydon, senior national co-ordinator for counter-terrorism policing, said: ‘At this point in our investigation, we believe the Skripals first came into contact with the nerve agent from their front door.
‘We are therefore focusing much of our efforts in and around their address. Those living in the Skripals’ neighbourhood can expect to see officers carrying out searches as part of this, but I want to reassure them that the risk remains low and our searches are precautionary.’
He thanked members of the public for their understanding, adding that officers will likely be in the neighbourhood for several months.
Mr Skripal and his daughter Yulia remain in a critical condition in hospital following the Novichok poisoning on March 4 in Salisbury, Wiltshire.
The spy’s niece, Viktoria Skripal said the prognosis for the pair ‘really isn’t good’, telling the BBC: ‘Out of 99% I have maybe 1% of hope.
‘Whatever it was has given them a very small chance of survival. But they’re going to be invalids for the rest of their lives.’
Britain has accused Russia of being behind the poisoning – allegations that have been fiercely denied by Moscow.
In turn, Russia has suggested that UK intelligence officers may have been involved in the poisoning, while also broadcasting multiple conspiracy theories.
The Kremlin allegation came as Moscow faced increasing global isolation, with at least 26 countries expelling a total of more than 130 suspected spies.
Britain insists there is no plausible alternative explanation for the attack, and has dismissed the series of suggestions emanating from Moscow as nonsense.
Around 250 counter terrorism detectives continue to work around the clock on the investigation, which is expected to continue for months.
Officers are examining more than 5,000 hours of CCTV footage and 1,350 exhibits that have been seized, while 500 witnesses have been identified.
The London Road cemetery, where his wife and son are buried, the Maltings shopping centre where Mr Skripal and his daughter were discovered unconscious, and the Ashley Wood compound have been handed back into the control of Wiltshire Police.