LONDON -- About 180 British military personnel are being sent to the scene of a nerve agent attack on a Russian ex-spy in southern England. Investigators are working to determine the source of the chemical. The military specialists will help remove contaminated items from the middle of the city of Salisbury, where Sergei Skripal, a Russian double agent, and his 33-year-old daughter Yulia were left in critical condition by the attack.

In total, 21 people sought medical treatment following Sunday's attack, which took place in broad daylight in Salisbury.

Russia's foreign minister expressed resentment at suggestions Moscow was behind the attack, for which the Kremlin has repeatedly denied any culpability.

The Metropolitan Police force said counterterrorism detectives had asked for military help "to remove a number of vehicles and objects from the scene" of the attack because "they have the necessary capability and expertise."

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The police said the health advice to the public remained the same; there is no broader risk.

Russian Foreign Minister Sergey Lavrov said Moscow was "ready to consider" lending a hand, "whether it's poisoning of some British subjects, whether it's rumors about interference in the U.S. election campaign."

"But in order to conduct such cases, it is necessary not to immediately run out on TV screens with unfounded allegations," Lavrov was quoted as saying by Russian state news agency Tass in the Ethiopian capital, Addis Ababa.

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 in a hospital in the city, 90 miles southwest of London.

A police officer who treated them at the scene is in serious condition.

The U.K. has vowed to take strong action against whoever was responsible for the "brazen and reckless" attack. ­­­British authorities say it's too soon to lay blame, but suspicions have fallen on Russia.

Those branded enemies of the Russian state have sometimes died mysteriously abroad, and the Skripal case echoes the death of Alexander Litvinenko, a former Russian agent who was poisoned in London in 2006 with radioactive polonium-210.

A British public inquiry found that Russia was responsible for Litvinenko's killing, and that Russian President Vladimir Putin probably approved it.

A former head of London's Metropolitan Police called Friday for new investigations into the deaths of 14 Russians in the U.K. amid suggestions they were targeted by the Russian state.

Former Commissioner Ian Blair, who led the London force when Litvinenko was fatally poisoned, told the BBC it is important to find out "whether there is some pattern here."

A BuzzFeed News investigation claimed U.S. spy agencies have linked 14 deaths to Russia, but U.K. police shut down the cases.

Russian media have mocked suggestions of Moscow involvement in the attack - but also noted that those who betray Russian seem to come to a bad end.

One anchorman on a Russian state television news show began a report on Skripal's poisoning with a warning to anyone considering becoming a double agent.

Channel One anchorman Kirill Kleimenov said in the Wednesday broadcast that he didn't wish death or suffering on anyone but wanted those "who dream of such a career" to know that traitors rarely live long.

"Alcoholism, drug addiction, stress and depression are inevitable professional illnesses of a traitor resulting in heart attacks and even suicide," Kleimenov said.