April 17 (UPI) -- The nerve agent used to poison former Russian double agent Sergei Skripal and his daughter Yulia was delivered in liquid form, the British government said.
The Department for Environment, Food and Rural Affairs said a small amount of the Soviet-era Novichok nerve agent was used in the March 4 attack. Of the nine locations that are potentially contaminated, Skripal's house had the highest concentration of the toxin, the department said.
The toxin will be present at the affected sites until they are decontaminated, the government said.
In a statement, the department said all nine sites need specialist cleaning, including The Maltings shopping center, Zizzi restaurant, the Ashley Wood compound, the Mill pub, two areas of Bourne Hill, Salisbury ambulance station, Amesbury ambulance station, the home of a police officer who fell sick after the incident and Skripal's home.
Skripal's home and the Mill remain part of the ongoing investigation.
Officials won't reopen sites until they've been approved by the government's decontamination science assurance group. Ian Boyd, the department's chief science adviser who is leading the decontamination effort, said that could take months.
"Our approach is based on the best scientific evidence and advice to ensure decontamination is carried out in a thorough and careful way," Boyd said. "Our number one priority is making these sites safe for the public, so they can be returned to use for the people of Salisbury."
Sergei Skripal, 66, and Yulia Skripal, 33, were found slumped on a bench at a Salisbury shopping area on March 4 after the poisoning. Yulia Skripal has since been discharged and Sergei Skripal is in stable condition.
British officials, including Prime Minister Theresa May and Foreign Secretary Boris Johnson, blamed Russia for the attack. In retaliation, British and U.S officials expelled Russian diplomats from their embassies -- and Russia answered by expelling British and U.S. diplomats from Moscow.
The Organization for the Prohibition of Chemical Weapons confirmed last week that the Russian-made Novichok nerve agent was used in an attack Skripal and his daughter.
The Russian government has denied involvement in the attack.