The British government was set to hold an emergency top-level meeting Wednesday to discuss the apparent poisoning of a former Russian spy, as the country's top diplomat warned that the UK would respond "robustly" if the Kremlin was found to be involved.
UK Home Secretary Amber Rudd will chair the cabinet-level COBRA meeting to discuss the investigation into the case of Sergei Skripal -- a former Russian military official convicted of spying for the UK -- and his daughter, who were found unconscious on a bench in southern England on Sunday.
Due to the "unusual circumstances" of the case, London's Metropolitan police said its counter-terrorism unit had taken over the investigation from local police.
The high-level meeting comes after UK Foreign Secretary Boris Johnson said Tuesday that while he is not "pointing fingers, as we cannot," Britain would make it clear to governments "around the world that no attempts to take innocent lives on UK soil will go unpunished."
"If evidence emerges of state responsibility, the government will respond appropriately and robustly," he said.
Skripal, 66, and his daughter Yulia, 33, are critically ill in a UK hospital after passing out on a shopping center bench in the English city of Salisbury on Sunday, having suffered "suspected exposure to an unknown substance," police said.
Skripal was convicted in Russia of spying for Britain before being granted refuge in the UK after a high-profile spy swap between the United States and Russia in 2010.
He is believed to have lived in the UK since his release from Russian custody in 2010. His daughter was visiting from Russia.
Skripal's daughter is thought to be one of the few members of his immediate family still alive after his wife, Lyudmila, and son Alexander died in recent years.
Johnson's comments Tuesday drew an acerbic response from Russia's embassy in London, which released a statement saying it "looks like the script of yet another anti-Russian campaign has already been written."
In a statement, the embassy said it was "impressed" by Johnson's comments, saying he spoke "in such a manner as if the investigation was already over and Russia was found responsible for what had happened in Salisbury."
"We regret that instead of a proper official clarification on the issue the foreign secretary chose to threaten Russia with retribution," it read.
Johnson also called into question the UK's involvement in the 2018 World Cup, which kicks off in Russia in 100 days. UK Foreign Office representative later clarified those comments, stating that Johnson was referring to the UK's diplomatic representation at the event, rather than the English football team, which has qualified for the tournament.
Salisbury, perhaps best known as the stopping-off point for tourists visiting nearby Stonehenge, has become the unlikely center of an extensive police probe into the mysterious circumstances around how Skripal and his daughter fell ill.
Authorities have declined to name the substance to which the pair are suspected to have been exposed to, and it's believed tests are still being carried out.
Investigators wearing protective fear and masks were seen Tuesday evening inside Zizzi's restaurant near Salisbury city center, where the Skripals were found. It's one of a number of sites that are being examined in the area.
A small number of emergency services personnel were treated immediately after attending a scene of suspected contamination, local police said Tuesday. One of them remains in the hospital.
'Echoes' of prior state-sponsored killing
Johnson also said Tuesday that there are "echoes" in this case 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 Mayfair hotel of London in 2006.
A detailed UK inquiry later concluded that Russian President Vladimir Putin probably approved the operation by Russian agents to kill Litvinenko. At the time, the Russian Foreign Ministry dismissed the UK investigation as politically motivated.
On Tuesday, the opposition Labour Party's home affairs spokeswoman Yvette Cooper called for a review of all suspicious deaths in the UK that could be linked to the Russian state, by Britain's National Crime Agency (NCA) or a similar body.
In a letter to Rudd, the Home Secretary, Cooper referred to media reports which suggested 14 deaths of Russians in the UK had raised concerns.
"Surely a review of all these cases and the decisions made on them, in light of this further reported evidence, carried out by the NCA, would be a prudent course of action to ensure that -- as the Foreign Secretary said today -- no attempt on an innocent life on British soil should go uninvestigated or unpunished," Cooper said.
Who is Sergei Skripal?
Skripal arrived in the UK as part of an elaborately choreographed spy swap conducted by the United States and Russia in which the two countries exchanged agents on chartered planes on the runway at an airport in Vienna, Austria.
According to previous reports by Russia's state news agency RIA Novosti, Skripal was sentenced to 13 years in prison in 2006 for spying for the UK.
It quoted Russia's Federal Security Service, or FSB, as saying Skripal, a former Russian military intelligence colonel, was convicted for "treason in the form of espionage" and that he had been recruited by Britain's MI6 intelligence service.
The report said he had spied for Britain as an intelligence officer during the 1990s and he had continued to communicate with MI6 after his retirement in 1999.
Russian court officials said Skripal had received at least $100,000 for his collaboration with MI6, RIA Novosti reported.