Sergei Skripal and his daughter Yulia first came into contact with a nerve agent at the former Russian spy's home in Salisbury, police believe.
As a result of detailed forensic and scientific examination, specialist detectives identified the highest concentration of the nerve agent Novichok on Mr Skripal's front door.
Around 250 counter terrorism detectives continue to work around the clock on the investigation, supported by a range of experts and partners.
Police were called by members of the public on March 4 after Mr Skripal, 66, and 33-year-old Yulia, were spotted slumped on a bench near the Maltings shopping centre.
The pair remain unconscious and in critical but stable conditions.
Officers are trawling 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.
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."
The attack on Mr Skripal and his daughter has brought Anglo-Russian diplomatic relations to one its lowest points in years.
In the wake of the incident in Salisbury, the British government expelled 23 Russian diplomats. In retaliation, the Kremlin announced it would be sending home 23 British officials home.
Foreign secretary Boris Johnson accused Russia of trying to conceal "the needle of truth in a haystack of lies" over the poisoning.
In a major foreign policy speech on Wednesday night, Mr Johnson said the targeted attack had crystallised a "global wave of revulsion" against Russia and that is should act as a wake-up call for Moscow.
In his speech at the Mansion House Easter Banquet the foreign secretary said: "Never before has there been a collective expulsion of Russian diplomats on the scale that we have seen over the last few days.
"It seems clear that the Kremlin underestimated the strength of global feeling.
"I do not for one moment believe that this global wave of revulsion has been prompted solely by Salisbury, let alone a sentimental love or affection for the UK.
"It wasn't about us. It was about all of us and the kind of world we want to live in.
"Because, I believe these expulsions represent a moment when a feeling has suddenly crystallised.
"When years of vexation and provocation have worn the collective patience to breaking point, and when across the world - across three continents - there are countries who are willing to say enough is enough."
More than 20 countries have now expelled Russian diplomats over the Skripal poisoning despite the Kremlin continuing to deny any involvement.