Detectives investigating the poisoning of Sergei and Yulia Skripal say they believe they first came into contact with the nerve agent Novichok at their home, the Metropolitan Police said.
Specialists have also identified the highest concentration of the nerve agent Novichok as being on the Skripals' front door, the Metropolitan Police added.
Deputy Assistant Commissioner Dean Haydon, Senior National Coordinator 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.
"I'd also like to thank the local community for their continued support and understanding. The unique circumstances of this investigation means that officers are likely be in the area for several weeks and months."
In a statement, the Met Police said: "Around 250 counter terrorism detectives continue to work around the clock on the investigation, supported by a full range of experts and partners.
"Officers continue to trawl through more than 5,000 hours of CCTV and examine over 1,350 exhibits that have been seized. Around 500 witnesses have been identified and hundreds of statements taken.
"Traces of the nerve agent have been found at some of the other scenes detectives have been working at over the past few weeks, but at lower concentrations to that found at the home address."
Meanwhile, a judge may end up having to make the politically-sensitive decision over whether to maintain life support for the Skripals, experts have said.
A friend of the poisoned spy says he believes the critically ill pair are supported by machines in a Salisbury hospital and his niece has claimed they only have a 1 per cent chance of survival.
Their deaths would have global repercussions, further ratcheting up the tensions between Russia and the West over who was responsible for a nerve agent attack.
If, as friends and family suggest, the Skripals are on life support, the Court of Protection could be called in to rule on treatment were their condition to deteriorate.
Zak Golombeck, a senior human rights lawyer at Slater and Gordon, said that as the court had already been involved in the case over the taking of blood samples, it was likely to be involved in future decisions over their care.
Doctors treating them would likely be encouraged to consult the Skripal's family members in Russia and the Official Solicitor, the British body which represents people who lack mental capacity, he said.
A judge at the Court of Protection, where issues relating to people who lack the mental capacity are considered, gave doctors permission to take blood samples from the Skripals earlier this month.
Professor Anthony Glees, a security expert University of Buckingham, told the Daily Mail: "Mr Skripal's death would be a family tragedy, and doubly so if his daughter also dies, but it would also be of international significance.
"The Government will then have to tell us what was the secret intelligence which the Foreign Secretary was basing his claim that it was "overwhelmingly likely" that Putin was behind it, or the Government could be in big trouble.
"Because Putin will play it for all it's worth.
"In short, the British Government has no interest in seeing his life support system being turned off any time soon."
Skripal's best friend, Ross Cassidy, has said that the double agent and his daughter should be allowed to die.
It was also revealed that Skripal's mother had not been told of the incident, while family believe there is just a 'one per cent chance' of the pair pulling through.
He told Sky News: "Quite frankly, what future have they got? I don't know the properties of this weapon that was used on them and my guess is they are probably being kept alive by artificial means and what life will they have if they survive?
"We've already been told they will be severely mentally impaired and I don't think they would want that. I think death would probably be merciful."
It comes as Skirpal's niece said her uncle and cousin have only a slim chance of surviving.
Viktoria Skripal said the prognosis for the former Russian double agent and his daughter Yulia "really isn't good".
She told the BBC: "Out of 99 per cent I have maybe 1 per cent 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. The first priority was to protect our granny so that she wouldn't hear or find out anything."
- additional reporting PA, AAP, Telegraph Group Ltd