Britain has deployed specialist troops to remove potentially contaminated objects from the site where former Russian spy Sergei Skripal and his daughter were found unconscious after a suspected nerve-agent attack.
Skripal, 66, and his daughter Yulia, 33, have been in hospital since they were found on a bench outside a shopping center in the southern English city of Salisbury on March 4.
Home Secretary Amber Rudd, who visited the city on March 9, including the area around the bench, said they were both still in very serious condition.
Britain's Defense Ministry and the police said some 180 troops, including some chemical experts, had been sent to Salisbury to remove ambulances and other vehicles involved in the incident as well as other objects.
Counterterrorism police, who are leading the investigation, said in a March 9 statement, "The public should not be alarmed," adding that "military assistance will continue as necessary during this investigation."
Britain has said it will respond appropriately if evidence shows Moscow was behind the incident, which police are treating as attempted murder. Moscow has denied involvement and asserted that anti-Russian hysteria is being whipped up by the British media.
British authorities say they have identified the substance as a rare nerve agent, which experts say should help identify the source, but they have not named it publicly.
Experts say the fact that officials have identified it as a rare nerve agent means it was probably not sarin or VX, which are relatively common.
Nerve agents are highly toxic chemicals that disrupt the nervous system and shut down bodily functions.
Both Sergei and Yulia Skripal remain unconscious, in critical but stable condition, while police officer Nick Bailey, who responded to the incident and was also harmed by the substance, remains in serious condition but is now able to speak.
Police said a total of 21 people had been treated in hospital following the incident, but only Skripal, his daughter, and Bailey remain hospitalized. Officials did not immediately explain how the others might have been exposed to the substance.
Britain’s chief medical officer, Sally Davies, said on March 8 that the general public was not necessarily at high risk, but experts say nerve agents are dangerous and extremely volatile.
A spokesman for British Prime Minister Theresa May said the poisoning was an "appalling and reckless crime.”
Rudd on March 8 told Parliament that the use of a nerve agent on British soil "is a brazen and reckless act" but that Britons must avoid speculating on who was behind it.
"This was attempted murder in the most cruel and public way," said Rudd, Britain's top police official. "But if we are to be rigorous in this investigation, we must avoid speculation and allow the police to carry on their investigation."
Rudd said British authorities "will respond in a robust and appropriate manner once we ascertain who was responsible."
"We are committed to do all we can to bring the perpetrators to justice, whoever they are and wherever they may be," she said.
Skripal, a retired Russian military intelligence colonel, was one of four prisoners released by Moscow in 2010 in exchange for 10 Russian sleeper agents in the United States, as part of a swap that included high-profile spy Anna Chapman.
Skripal had been arrested in Moscow in December 2004 and convicted by a Moscow military court in August 2006 of "high treason in the form of espionage."
He was found guilty of passing the identities of Russian intelligence agents working undercover in Europe to Britain's Secret Intelligence Service, or MI6, in return for $100,000.
Russia's Federal Security Service (FSB) alleged he had begun working for MI6 while serving in the army in the 1990s.
The incident in Salisbury has drawn comparisons with the 2006 death of former Russian security agent Aleksandr Litvinenko in London.
A British inquiry has concluded that the Russian government was behind Litvinenko's death and that President Vladimir Putin "probably approved" the killing. Russia has denied involvement.
Foreign Secretary Boris Johnson has warned that any involvement of a foreign government in the incident in Salisbury would not go unpunished.
Johnson told Parliament on March 6 that Britain might step up sanctions against Russia if it finds that Moscow was involved. He also suggested that Britain could reconsider the participation of its officials in the soccer World Cup in Russia this summer.
Russian Foreign Minister Sergei Lavrov said on March 9 that Britain's warnings of retaliation if it is proven Russia was behind the poisoning of a double agent are propaganda and not serious.
Speaking to reporters during a visit to Ethiopia, Lavrov said Russian officials have seen no concrete evidence or "facts" about what happened to Skripal and his daughter.
"All we see are TV reports...saying that if it is Russia, there will be a response that Russia will remember forever. This is not serious, it is pure propaganda and whipping up hysteria," Lavrov said.
He reiterated Moscow's offer for assistance in the investigation, but said that "in order to have a serious conversation...you have to use the official channels."
"If Russia's help is genuinely needed, we will be willing to consider this possibility if we have the necessary information," Lavrov said.