Former Russian spy Sergei Skripal and his daughter were deliberately poisoned by a nerve agent in England over the weekend, UK police said Wednesday.
Metropolitan Police Assistant Commissioner Mark Rowley told reporters that police were treating the incident as "attempted murder by a nerve agent," though he declined to elaborate on the specific nerve agent that was believed to have been used.
Skripal -- a former Russian military official convicted of spying for the UK -- and his daughter Yulia are critically ill in a UK hospital after passing out on a shopping center bench in the southern English town of Salisbury on Sunday.
A police officer who was one of the first to arrive on the scene on Sunday has also fallen ill and is now in serious condition in hospital, Rowley said Wednesday.
Nerve agents are highly poisonous chemicals that prevent the body's nervous system from functioning properly, according to the Centers for Disease Control and Prevention. Exposure to large doses can result in death.
On Tuesday, UK Foreign Secretary Boris Johnson said that there were "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 2006 in a hotel in London.
A detailed UK inquiry later concluded that Russian President Vladimir Putin probably approved the operation by Russian agents to kill Litvinenko. The Russian Foreign Ministry dismissed the inquiry as politically motivated.
Johnson said Tuesday that he was "not pointing fingers" but 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 added.
Johnson's comments drew an acerbic response from Russia's embassy in London, which released a statement Tuesday saying it "looks like the script of yet another anti-Russian campaign has already been written."
Russia had not received an official request from British authorities to assist in the investigation, Russian Foreign Ministry spokeswoman Maria Zakharova told reporters Wednesday.
Skripal, 66, was convicted in Russia of spying for Britain before he was granted refuge in the UK after a high-profile spy swap in 2010 between the United States and Russia.
He is believed to have lived in the UK since his release from Russian custody in 2010. His daughter, 33, was visiting from Russia.
A woman who saw Skripal and his daughter earlier Sunday described the couple as appearing "out of it," as if they had "been taking something quite strong."
The counter-terrorism unit of London's Metropolitan Police has taken over the investigation from local police, with hundreds of counter-terrorism police officers now assigned to the investigation.
Police are concurrently working with a range of scientists and the Department of Health and said there was no evidence that points to a wider public health risk.