Spy poisoning: Military remove ambulances and police cars for testing

Russia has rolled out a robot tank

Sergei Skripal and his daughter Yulia remain unconscious in a critical but stable condition following the attack on Sunday in the sleepy south-western English city of Salisbury.

Ms Rudd said the pair are in a "very serious" condition five days after they were discovered slumped on a bench in Salisbury, Wiltshire.

The Ministry of Defence has sent in 180 specialist personnel and equipment to support the police investigation.

Former London Police Chief Ian Blair said Friday that the police officer who is seriously ill had visited Skripal's house - suggesting the nerve agent may have been delivered there.

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". In 2010, President Dmitry Medvedev pardoned him, after which, in the framework of exchange of spies with the U.S., he was sent to the United Kingdom, where he was granted asylum.

Prime Minister Theresa May has said Britain will react with the appropriate response if a state was found to be behind the murder attempt.

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 vehicle park.

"Scientific tests by government experts have identified the specific nerve agent used which will help identify the source", they said.

Last week would have been Mr Skripal's son's birthday, so it's likely Mr Skripal and his daughter, Yulia, visited, perhaps leaving flowers or other objects.

In a statement to members of Parliament, UK Home Secretary Amber Rudd refused to be drawn on who might have been responsible. "People are right to want to know who to hold to account but if we are to be rigorous in this investigation we must avoid speculation and allow the police to carry on their investigation".

Moscow's military court convicted Skripal of "high treason in the form of espionage" in August 2006 for passing information to Britain's MI6 intelligence service.

Russian Foreign Minister Sergey Lavrov said Moscow might be willing to help with the investigation, but he expressed resentment at suggestions that the Kremlin was behind the attack.

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.

Hardest hit himself Skripal and his daughter Julia.

A detailed United Kingdom inquiry later concluded that Russian President Vladimir Putin probably approved the operation by Russian agents to kill Litvinenko. Sky News reported that policemen in chemical protection suits took away flowers from the grave of Skripal's wife and son in Salisbury, which the poisoned relatives had brought there shortly before the incident.

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