London (CNN)Work is beginning in the English city of Salisbury to decontaminate potentially toxic "hot spots" linked to the nerve agent poisoning of former Russian double agent Sergei Skripal and his daughter Yulia.
Salisbury cleanup targets toxic 'hot spots' after Skripal poisoning
Residents were briefed Thursday evening by government and local officials on the cleanup operation in nine locations across the city, according to Britain's Press Association news agency.
Police believe the pair first came into contact with the substance, identified by the Organisation for the Prohibition of Chemical Weapons as a Novichok agent, at Sergei Skripal's home. The highest concentration of the substance was found on the front door.
A number of sites around the city -- including a pub and restaurant visited by the Skripals -- have been cordoned off since the March 4 attack.
"We have to make an assumption that in certain circumstances there will be relatively high concentrations, probably in very, very specific locations, which could be at levels that could be toxic to individuals," Ian Boyd, chief scientific adviser of the Department for Environment, Food and Rural Affairs, told residents, according to the Press Association.