Salisbury - Britain's counterterrorism police took over an investigation on Tuesday into the mysterious collapse of a former spy and his daughter, now fighting for their lives.
The government pledged a "robust" response if suspicions of Russian state involvement are proven.
Foreign Secretary Boris Johnson said he wasn't yet accusing anyone of harming Sergei Skripal and his daughter, Yulia. The two Russians collapsed on Sunday on a bench in southern England after coming into contact with an unknown substance.
READ: Freed Russian spy reportedly sickened after mystery exposure
But he stressed that Britain would act - and possibly limit its participation in the upcoming soccer World Cup in Russia - if Moscow's hand is shown.
"I say to governments around the world that no attempt to take innocent life on UK soil will go either unsanctioned or unpunished," Johnson told lawmakers in the House of Commons.
Mystery
British counterterrorism specialists took control of the case from local police trying to unravel the mystery of why the two collapsed in Salisbury, 145km southwest of London.
Both Skripal, 66, and his 33-year-old daughter are in critical condition in the intensive care unit of Salisbury District Hospital, Wiltshire Police said.
Although authorities said they were keeping an open mind - and pointedly did not declare a terrorist incident - the affair evoked echoes of the death of former Russian agent Alexander Litvinenko, who was poisoned with radioactive polonium-210 in London in 2006.
A British inquiry found that Litvinenko's killing was committed by Russian agents, probably approved by President Vladimir Putin. Russia denied any involvement in Litvinenko's death, and denied suggestions of involvement in Skripal's collapse.
Russian Foreign Ministry spokesperson Maria Zakharova dismissed Johnson's remarks as "wild".
Police said it was too soon to jump to conclusions.
"I think we have to remember that Russian exiles are not immortal," Mark Rowley, the Metropolitan Police Service's assistant commissioner for counterterrorism, told the BBC.
"They do all die and there can be a tendency for some conspiracy theories. But likewise we have to be alive to the fact of state threats, as illustrated by the Litvinenko case."
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