Britain's counterterrorism police took over the investigation into the sudden and severe illness of former Russian spy Sergei Skripal and his daughter as media reported that Skripal's son died last year of unknown causes on a visit to Russia.
Scotland Yard announced that its counterterrorism unit would take charge due to the case's "unusual circumstances" after Foreign Secretary Boris Johnson warned that any involvement of a foreign government in the incident would not go "unpunished."
Britain's stepped-up response to the still-unexplained events that put Skripal, 66, and his daughter Yulia, 33, into intensive care at a hospital in Salisbury came as the BBC quoted Skripal's relatives as saying that some Skripal family members died in recent years in mysterious circumstances.
Skripal's relatives told the BBC Russian Service that the ex-spy believed that "Russian special services might come after him at any time." The BBC did not elaborate.
Skripal's son Sergei, 44, died on a visit to Russia last year of an unknown illness, the Times reported, while the Guardian reported that Skripal's wife died from cancer shortly after her arrival in Britain in 2012.
The Times reported that Yulia Skripal lived in Britain in 2010 after her father was released in a spy swap with Russia, but she later moved back to Moscow and was working for PepsiCo Russia. She arrived back in Britain to visit her father last week, according to the Times.
Emergency Committee Meeting
A meeting of London's emergency committee was scheduled for March 7 to discuss what happened to the Skripals.
Speaking to parliament on March 6, Johnson said Britain might step up sanctions against Russia if it found that Moscow was involved in the incident in the southern English city of Salisbury.
"We don't know exactly what has taken place in Salisbury, but if it's as bad as it looks, it is another crime in the litany of crimes that we can lay at Russia's door," Johnson said.
Skripal and his daughter were found unconscious on a bench outside a shopping mall in Salisbury after stopping at a nearby restaurant and pub. Police suspect that exposure to an as-yet unidentified substance caused their illness.
"Should evidence emerge that implies state responsibility, then Her Majesty's government will respond appropriately and robustly," Johnson told parliament.
"I am not now pointing fingers," he said. "I say to governments around the world that no attempt to take innocent life on U.K. soil will go either unsanctioned or unpunished."
He indicated that strong evidence of Russian government involvement could lead to new punitive measures against Moscow, which is under European Union and U.S. sanctions over its aggression in Ukraine and other actions.
"If the suspicions...prove to be well-founded, then it may very well be that we are forced to look again at our sanctions regime and other measures that we may seek to put in place," Johnson said.
"It is clear that Russia, I'm afraid, is now in many respects a malign and disruptive force, and the U.K. is in the lead across the world in trying to counteract that activity," he said.
Russian Foreign Ministry spokeswoman Maria Zakharova criticized Johnson's comments, calling them "wild."
The unexplained incident in Salisbury swiftly drew comparisons with the death of Aleksandr Litvinenko, a former Russian security agent who fell ill and died in London in November 2006 after ingesting radioactive polonium-210.
A British inquiry concluded that the Russian government was behind Litvinenko's death and that President Vladimir Putin "probably approved" the killing. Russia has denied any involvement.
Skripal is a former colonel in Russia's GRU military intelligence agency who was convicted of passing state secrets to Britain in 2006 but was released from prison -- and sent to the West -- in a spy swap in 2010.
Putin's spokesman said that Russia has "no information" on what could have led to the hospitalization of Skripal and his daughter, which he called a "tragic situation."
Asked to respond to British media speculation that Russia had poisoned Skripal, Dmitry Peskov said: "It didn't take them long."
The Russian Embassy in London said the incident was being used to demonize Russia.
Parts of the Salisbury shopping center where the Skripals were found remained sealed off on March 6. One member of Britain's emergency services that responded to the incident also took ill and remained in the hospital.
Police vehicles were also seen at what was listed as Skripal's home, which is reported not far from the shopping center.
Scientists at Porton Down -- the U.K.'s secret weapons research facility in Wiltshire -- were studying the "unknown substance" believed to have caused the Skripals' illness, officials said.
Local media reported that emergency services suspect the powerful synthetic opiate fentanyl may have been involved.
Skripal was 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.
With reporting by BBC, The Times, The Guardian, Reuters, AP, AFP, dpa, and Press Association