The nerve agent used to poison Sergei Skripal and his daughter Yulia in Salisbury was delivered in a liquid form, the government has said.
Nine sites in the Wiltshire city were potentially contaminated following the 4 March attack, carried out with a “very small amount” of a military-grade novichok, an official from the Department for Environment, Food and Rural Affairs (Defra) revealed.
The highest concentration was found at the Russian former spy’s house, he told a press briefing in Salisbury.
“In this instance direct contact is required for a person to be poisoned – only a small proportion of the material is transferred and the substance is diluted in each secondary and tertiary contact,” the official added.
“The class of nerve agent does not produce significant vapour or gas and can only move between sites by direct transfer by a contaminated person or item.”
The official confirmed that novichok “doesn’t just disappear”, meaning that if the substance was detected in the ongoing police investigation it will still be present.
Contaminated parts of the city will be sealed off behind high security barriers for months as work continues to make the city safe.
Skripal attack aftermath – in pictures
The “primary site of contamination” is Mr Skripal’s house and eight other sites are believed to have been contaminated, including the Zizzi restaurant and The Mill pub the former double agent visited with his daughter, police and ambulance stations and Detective Sergeant Nick Bailey’s home.
Public Health England (PHE) reassured members of the public that the risk to health remained low and said sites would be chemically cleaned and retested before being reopened.
The cemetery where the remains of Mr Skripal’s wife and son are buried was found not to be contaminated and has reopened to the public.
Police have said previously that the highest concentration of the nerve agent was found on the former double agent’s front door.
The 66-year-old and his daughter, 33, were left fighting for life following the suspected assassination attempt and were initially feared to have little chance of survival. But their conditions have improved and Ms Skripal has since been discharged from hospital.
Her father remains in hospital in a stable condition.
The Maltings shopping centre and a Zizzi restaurant where the Skripals dined on the day they fell ill are among the nine locations identified as requiring “some level of specialist cleaning”, said Defra. Three of the contaminated sites are in the city centre.
Public Health England stressed the risk to the public was low.
“All remaining potentially contaminated sites will remain secured and the current scientific assessment is that the remainder of Salisbury is safe for residents and visitors,” said Defra.
Twenty-one people received medical treatment for exposure to the novichok in the days following the attack.
Several were police officers, including DS Bailey, who fell seriously ill and was treated in intensive care before being discharged two weeks later.
Four independent laboratories have concluded the chemical used in the attack was a novichok-class nerve agent, which were originally developed by the Soviet Union, the Organisation for the Prohibition of Chemical Weapons said last week.
Their conclusions match that of UK scientists at the Porton Down defence laboratory, which the government said proved Moscow was culpable for the attack after allegedly spying on the Skripals for at least five years.
The Foreign Office said there was ”no alternative explanation about who was responsible – only Russia has the means, motive and record”, and demanded the Kremlin “give answers”.
Russia continues to deny any involvement in the attack.