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 Sunday's incident as "attempted murder by a nerve agent," though he declined to elaborate on the specific substance 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 after passing out on a shopping center bench in the southern English town of Salisbury.
A police officer who was one of the first to arrive on the scene also has fallen ill and is in serious condition at a hospital, Rowley said Wednesday.
'Echoes' of the past
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 Wednesday, Foreign Secretary Boris Johnson warned that Britain would "respond robustly" if the attack was found to be the work of a foreign power.
Johnson had 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's Tuesday comments 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."
Russia had not received an official request from British authorities to assist in the investigation, Foreign Ministry spokeswoman Maria Zakharova told reporters.
Europol's executive director Rob Wainwright described the incident as "outrageous affront to our security in Europe and our way of life" in a tweet, but cautioned not "jumping to any conclusions" as to who was responsible.
The UK government convened an emergency cabinet-level meeting to discuss the investigation into the Salisbury incident.
Skripal's English refuge
Skripal, 66, is believed to have lived in the UK since his release from Russian custody in 2010.
He 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.
His daughter Yulia, 33, is thought to be one of the few members of the former spy's immediate family still alive after his wife and son died in recent years. She was visiting him from Russia at the time of the incident.
Police officers outside the Zizzi restaurant in Salisbury which has been closed following the incident.
A woman who saw Skripal and his daughter in Salisbury town center on Sunday described the pair as appearing "out of it," as if they had "been taking something quite strong."
Police have called on anyone who visited the area on Sunday, including Zizzi restaurant or the nearby Bishop's Mill pub -- two sites that are being examined -- to come forward with any information that might help them piece together what happened.
Local convenience store manager Ebru Ozturk had seen Sergei Skripal at the Bargain Stop shop in Salisbury just five days before the incident. She told CNN that he was a "kind customer" who would usually come in once a week and buy Polish-smoked bacon and scratch-and-win lottery cards.
"His wife died a few years ago. He was feeling bit sad. He started to get used to living on his own after the wife died," Ozturk said.
"He is (a) regular customer, he is so kind and he seems to me an educated person. Very polite," said Ozturk. "I don't talk too much to the customers, but he was, you know, one of the very kind customers."
CCTV footage showed Skripal talking to Ozturk and buying items at the store on February 27, five days before he was apparently poisoned.
The counterterrorism 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.