Former Russian spy Sergei Skripal and his daughter were deliberately poisoned by a nerve agent, UK police say, significantly increasing the likelihood that a foreign state was behind the attack.
Mark Rowley, head of the counter-terrorism division of London's Metropolitan Police, told reporters that Sunday's incident was being treated as attempted murder. He revealed that one of the first police officers on the scene had fallen sick and was seriously ill in hospital.
The developments mean that the British government must confront the suspicion that elements linked to the Kremlin carried out a brazen attack with a deadly agent in the middle of a rural English city, two weeks before President Vladimir Putin seeks re-election for a new six-year term.
If a Moscow link were proved, it would plunge relations between the West and Russia into a new low, and would call into question the British government's ability to protect residents of the UK at home. Rowley said officers from Wiltshire police, the local force, were providing support to the sick officer and his families, but insisted there was no evidence of a wider threat to public health.
Skripal -- a former Russian military official convicted of spying for the UK -- and his daughter Yulia remain critically ill in hospital after being found unconscious on a shopping center bench. Police said they believe the pair were the specific targets of the attack.
"This being treated as a major incident involving attempted murder by administration of a nerve agent," Rowley said. Describing the investigation as fast-moving, police urged members of the public to get in touch with any information.
Rowley said UK government experts had identified the specific nerve agent used in the attack, but declined to say what it was.
The executive director of Europol, the EU's law enforcement agency, described the incident as "outrageous affront to our security in Europe and our way of life," 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 on Wednesday.
'Echoes' of the past
Confirmation that a nerve agent was used in the Salisbury attack considerably narrows the range of suspects. Nerve agents, highly poisonous chemicals that that prevent the body's nervous system from functioning properly, are rarely used outside the battlefield and require considerable expertise to develop. Exposure to potent doses can result in death.
Sarin was used in the 1995 attack on the Tokyo subway, carried out by the Aum Shinrikyo doomsday cult, which killed 13 people and injured 5,500 others. Another nerve agent, VX, was used to kill Kim Jong Nam, the half brother of North Korean leader Kim Jong Un, at Kuala Lumpur international airport in February 2017.
On Wednesday, UK Foreign Secretary Boris Johnson warned that Britain would "respond robustly" if the Salisbury 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.
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.
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."