Police investigating the poisoning of a former Russian spy and his daughter have revealed the pair first came into contact with the nerve agent at their home in Salisbury.

The highest concentration of the nerve agent Novichok was found on the front door of Sergei Skripal’s home, police said on Wednesday. Both he and his daughter, Yulia, remain in a critical condition in hospital.

Police said they will continue to focus their enquiries around the home for the coming weeks – and possibly months – as the investigation continues.

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.”

Theresa May said on Wednesday that the Skripals “may never fully recover” from the attack, as she revealed that 130 people in Salisbury could have been exposed to the deadly nerve agent Novichok.

Skripal, 66, and his 33-year-old daughter, were found slumped on a bench in Wiltshire earlier this month.

Detective sergeant Nick Bailey, who was also exposed to the nerve agent, said “normal life for me will probably never be the same” after he was discharged from hospital last week. 

The Met Police said that about 250 counter terrorism detectives are working on the investigation.

There are more than 5,000 hours of CCTV to look through and more than 1,350 exhibits have been seized.

About 500 witnesses have been identified and hundreds of statements taken, police said.

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.

Police said that a number of sites in Salisbury under investigation are being handed back to Wiltshire Police. These sites include the London Road Cemetery, the Ashley Wood compound and The Maltings.

Wiltshire Police deputy chief constable Paul Mills said: “I would like to reiterate our thanks to the people of Salisbury for their patience and understanding.

“We are very aware of the impact the closure of these scenes has had on the local community and businesses and we are very grateful for the support and cooperation that has been, and continues to be, shown whilst the police investigation and recovery process takes place.”

The prime minister said on Wednesday that the government had evidence that Russia had explored ways of exporting nerve agents over the past decade, most likely for the purposes of assassinations.

May also said that Russia has since advanced 21 different arguments to try and distance itself from the attacks.

More than 100 Russian intelligence officers have been expelled from Western nations following the nerve agent attack.