British police say a nerve agent was used to try to kill former Russian spy Sergei Skripal and his daughter in the southwestern city of Salisbury.
"This is being treated as a major incident involving attempted murder, by administration of a nerve agent," Mark Rowley, head of Counterterrorism Policing, said on March 7.
"I can also confirm that we believe that the two people who became unwell were targeted specifically," he added, without giving details of the substance used.
Skripal, 66, and his daughter Yulia, 33, were found unconscious on a bench outside a shopping mall in Salisbury after stopping at a nearby restaurant and pub, and remain “critically ill,” Rowley said.
A police officer who also became ill after attending the scene is in a “serious condition in hospital," he added.
Nerve agents such as sarin are highly toxic chemicals that disrupt the nervous system and shut down bodily functions.
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.
Earlier on March 7, Britain’s Home Secretary Amber Rudd called for “cool heads” as the investigation into the Salisbury incident moves forward.
She said authorities would “respond to evidence, not to rumor” in the incident, which has drawn comparisons with the 2006 death of former Russian security agent Aleksandr Litvinenko in London.
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.
On March 6, Foreign Secretary Boris Johnson warned that any involvement of a foreign government in the incident would not go "unpunished."
Speaking to parliament, Johnson said Britain might step up sanctions against Russia if it finds that Moscow was involved in the incident.
"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," he said.
The Russian Foreign Ministry on March 7 accused British politicians and journalists of using the case to drive "anti-Russian" sentiment and disrupt relations between London and Moscow.
"What happened to Skripal has been immediately used to further incite an anti-Russian campaign in Western media," spokeswoman Maria Zakharova said.
She had earlier characterized Johnson's comments as "wild" and "words of savagery."
Scotland Yard has announced that its counterterrorism unit would take charge of the investigation due to the case's "unusual circumstances."
Britain's stepped-up response to the Skripal incident came as the BBC quoted Skripal's relatives as saying that some Skripal family members died in recent years under 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.
"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.
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."
With reporting by BBC, The Times, The Guardian, Reuters, AP, AFP, dpa, and Press Association