Detectives investigating the attempted murders of Russian double agent Sergei Skripal and his daughter Yulia Skripal have said they believe the pair were poisoned with a nerve agent at the front door of his Salisbury home.

Specialists investigating the poisoning of the the Skripals have found the highest concentration of the nerve agent on the front door at the address, police said.

Counter-terrorism detectives will continue to focus their inquiries on the home address for the coming weeks, and possibly months, after the father and daughter were found unconscious on a park bench in Salisbury earlier this month.

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Local police have retaken control of The Maltings shopping centre, where the Skripals were first discovered, and London Road cemetery from counter-terrorism detectives, where officers focused their investigation into the nerve agent attack in previous weeks.

More than 130 people could have been exposed to the chemical weapon in the aftermath of the poisoning in Salisbury, which the UK government believes was committed by the Russian state.

In response to the poisoning, more than 150 Russian officials have been expelled from more than 25 countries, and the UK government is considering further measures to punish Russia, including a ban on the City of London from selling Russian sovereign debt.

Public health experts are still working to establish whether the nerve agent attack presents a long term risks to Salisbury’s residents, which will receive a £1m support package from central government to help recover.

Deputy assistant commissioner Dean Haydon, the senior national coordinator for counterterrorism 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.”

About 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 more than 1,350 exhibits that have been seized. Approximately 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 than that found at the home address.