Sergei Skripal, the Russian ex-spy and his daughter, Yulia, who were poisoned by a nerve agent, first came into contact with the substance at their home, UK police have said.
Sergei and Yulia were found collapsed on a bench in Salisbury on March 4 and remain in a critical condition in hospital.
Counter terrorism police said their efforts were focussed around the Skripal’s address in the city, BBC News reports, adding that the risk to neighbours living near the Skripal’s remains low.
Specialists identified the highest concentration of the nerve agent to date at the front door of the Skripal’s house, police said in a statement.
Traces of the nerve agent had been found at some of the other sites in the city, but they were at lower concentrations.
About 250 counter terrorism officers continue to work on the case, examining 5,000 hours of CCTV footage and 1,350 items that have been seized.
Officers would continue working at the address over the next few weeks, and potentially months, Deputy Assistant Commissioner Dean Haydon said.
“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 added.
Forensic work at shops and at a cemetery in Salisbury is drawing to a close.
The Maltings shopping centre, the Ashley Wood compound, and the London Road cemetery have been handed back to Wiltshire Police from the national counter terrorism investigation, Deputy Chief Constable Paul Mills said.
Mr Skripal’s wife and son are buried in the cemetery, and the ex-spy and his daughter were discovered unconscious at the Maltings shopping centre.
Prime Minister Theresa May has said the chemical used in the attack was part of a group of nerve agents developed by Russia known as Novichok.
The prime minister has said there was “no alternative conclusion other than that the Russian state was culpable,” in the attack, and she has expelled 23 Russian diplomats from the UK.
In addition, at least 26 countries, including the US and European allies, are expelling more than 130 diplomats.
The Russian government has denied any involvement in the attack and has vowed to retaliate against the “provocative gesture”.
Russia has said it will expel 23 British diplomats from Moscow.