Sites that have been cordoned off include the city center bench where the two were found, a pub and restaurant they had visited earlier, Skripal's home, the graves of his wife, Lyudmila, and son, Alexander, and an industrial auto park. His son Alexander, 43, died previous year. Skripal's wife, Lyudmila, 59, died of cancer in 2012.
Suspicion is mounting that Russian Federation carried out the attempt on their lives as an act of revenge against the former intelligence officer, who was convicted in 2006 of selling state secrets to MI6.
British police have identified over 200 witnesses and are looking at more than 240 pieces of evidence in their investigation into a nerve agent attack on a Russian ex-spy and his daughter, the interior minister says.
About 180 troops including some with chemical expertise have been sent to the city to remove ambulances and other vehicles involved in the incident and other objects.
Analyst Mathieu Boulegue of the Chatham House think tank said it would be "extremely difficult" to pinpoint who committed the attack, and that the response could be a lot more complicated if it turned out that internal factions were responsible.
Earlier Friday, members of the armed forces removed a police auto used in the initial response from Salisbury district hospital.
British media reports suggested that Theresa May, the prime minister, could announce sanctions against Russian Federation as early as Monday if experts conclude that Russian Federation was behind the attack.
Officers in protection suits were seen stuffing items in several yellow barrels, believed to be flowers left by Mr Skripal before he was stricken by the nerve agent.
Detective Sergeant Nick Bailey, who was one of the first to come into contact with the contaminated pair, is also fighting for his life.
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He said: "There obviously are some indications the officer, and I'm very sorry that he has been injured, has actually been to the house, whereas there was a doctor who looked after the patients in the open who hasn't been affected at all". "There may be some clues floating around in here", he said of Skripal's house.
DS Bailey is among 21 people who received medical treatment following exposure to the nerve agent.
"A well-equipped lab and a very experienced analytical chemist can do it, but it's not the sort of thing a chancer doing kitchen-sink chemistry can get away with", chemical weapons expert Richard Guthrie said.
Russian Foreign Minister Sergei Lavrov gave his high-level assessment saying, "This is not serious; this is pure propaganda and the whipping-up of hysteria". "If you really need help from Russian Federation, then we will be ready to consider such a possibility, if we do have relevant data", he said, speaking in Addis Ababa, Ethiopia. British authorities say it's too soon to lay blame, but suspicions have fallen on Russian Federation.
Johnson had said the case had "echoes" of what happened to former Russian spy Alexander Litvinenko, who died a slow death after drinking tea laced with highly radioactive polonium-210 in a London hotel in 2006.
A British public inquiry found that Putin had probably ordered Litvinenko's murder.
Blair, the former London police chief, called for a review of the deaths of 14 Russians in the United Kingdom amid suggestions they were targeted by the Russian state. He was later released as part of a spy swap with the US.
"Alcoholism, drug addiction, stress and depression are inevitable professional illnesses of a traitor, resulting in heart attacks and even suicide", Channel One anchorman Kirill Kleimenov said in the Wednesday broadcast.
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The PKK is a threat posed not only to Baghdad but also to the Iraqi Kurdish administration in the north of the country, he added. The joint operation with Iraqi army may start after the elections in Iraq scheduled for May 12.