A former Russian spy and his daughter who were victims of a nerve agent attack first came into contact with the deadly chemical at his home, police said.
Ex-spy Sergei Skripal and his daughter Yulia remain in a critical condition in hospital following the Novichok poisoning on March 4 in Salisbury, Wiltshire.
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.
Britain has accused Russia of being behind the poisoning - allegations fiercely denied by Moscow.