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LONDON — Britain is sending 180 troops to the city where a Russian double agent was poisoned with a nerve agent, officials announced Friday.

Chemical weapons experts were involved in the investigation, a spokesperson from Britain's Defense Ministry told NBC News.

London's Metropolitan Police said in a statement that it had requested assistance from the military to remove a number of vehicles and objects from the site of the attack in Salisbury that left former spy Sergei Skripal, 66, and his 33-year-old daughter, Yulia, in critical condition.

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The military was asked to help as they have "the necessary capability and expertise."

Breaking news: Defence Secretary statement on the incident in #Salisbury pic.twitter.com/K5bBFv8CZ0 — Ministry of Defence (@DefenceHQ) March 9, 2018

Skripal, a former Russian military intelligence officer, was convicted in 2006 of spying for Britain and released in 2010 as part of a spy swap.

He had been living quietly in Salisbury, where he and his daughter were found unconscious on a bench Sunday. They are in critical but stable condition.

A police officer who treated them at the scene is in serious condition, and a total of 21 people have received medical treatment.

Police said Skripal and his daughter were deliberately targeted with the rare toxin. They said experts had identified the substance, which will help determine the source, but have not yet disclosed to the public what it was.

The attack drew comparisons to the assassination of ex-KGB agent Alexander Litvinenko, a critic of Russian President Vladimir Putin, who died in London in 2006 after drinking green tea laced with radioactive polonium-210.

Britain has said it will respond appropriately if evidence shows Russia was behind the poisoning. The Kremlin has denied any involvement in the incident and says anti-Russian hysteria is being whipped up by the British media.