(CNN) Detectives with London's Metropolitan Police said Wednesday they believe former Russian double agent Sergei Skripal and his daughter first came into contact with a nerve agent at Sergei Skripal's home, and police have identified the highest concentration of the nerve agent to date as being on the home's front door. 
 
 The pair were poisoned March 4 after being exposed to what British police believe was a military-grade nerve agent. They are hospitalized in critical condition. 
 
 Skripal and his daughter, Yulia, were found slumped on a bench in an outdoor shopping complex in Salisbury, England. They had no visible injuries, according to police. 
 
 They were near a number of restaurants, one of which was closed as part of the initial investigation. 
 
 Detectives now plan to focus their investigation around Sergei Skripal's Salisbury home for the coming weeks and possibly months, the Met Police said. Yulia Skripal was visiting her father. 
 
 Police also plan to search the surrounding area. 
 
 Yulia Skripal 
 
 "Those living in the Skripals' neighborhood can expect to see officers carrying out searches as part of this, but I want to reassure them that the risk remains low and our searches are precautionary," Deputy Assistant Commissioner Dean Haydon said in a statement. 
 
 Around 250 counterterrorism detectives are working around the clock on the investigation, the Met Police said, adding it is one of the largest and most complex investigations undertaken by British counterterrorism police. 
 
 Officers are examining more than 5,000 hours of closed-circuit television and more than 1,350 seized items, police said. They have identified around 500 witnesses and taken hundreds of statements. 
 
 Detectives have found traces of the nerve agent at other scenes in the past few weeks but at lower concentrations than at the home address, the Met Police said. 
 
 Surveillance footage shows Sergei Skripal shopping at a convenience store days before he was poisoned. 
 
 Diplomatic fallout 
 
 The UK has expelled 23 Russian diplomats over the poisoning of the Skripals, which it blames on Moscow. Russia denies it was involved and has suggested the UK could be behind it. 
 
 In a show of support for Britain, more than 20 other countries have kicked out Russian diplomats . Those countries include the United States, Canada, Australia and 18 European Union states. 
 
 In a speech Wednesday evening, British Foreign Minister Boris Johnson thanked the countries "from the bottom of my heart" for standing with Britain, saying they did it despite the risk of retaliation. 
 
 "I believe that these expulsions represent a moment when a feeling has suddenly crystallized, when years of vexation and provocation have worn collective patience to (the) breaking point, and when across the world -- across three continents -- there are countries who are willing to say enough is enough," Johnson said at an annual gathering of the diplomatic corps in London. 
 
 Russian Foreign Minister Sergey Lavrov said the United States pressured and blackmailed other countries to expel Russian diplomats. 
 
 British Prime Minister Theresa May speaks Monday in Parliament. 
 
 Moscow has already responded by expelling 23 British diplomats and shuttering British cultural institutions in Russia. Tuesday, Lavrov warned of further retaliation , saying, "Rest assured, we will respond," according to state media. 
 
 Russia's ambassador to Australia called the expulsions "unfair." 
 
 "It's rather like the beginning of 'Crime and Punishment,'" Johnson said. "We're all confident of the culprit, and the only question is whether he will first confess or be caught." 
 
 The Skripals 'may never fully recover' 
 
 According to the UK, the Skripals were exposed to a Soviet-era nerve agent known as Novichok, a colorless, odorless and tasteless liquid described as one of the world's most sophisticated and deadly chemical weapons. 
 
 While British officials have said they believe the public health risk is low, Prime Minister Theresa May has also said more than 130 people in Salisbury could have been exposed to the nerve agent. 
 
 May said Monday that doctors have indicated the Skripals may never fully recover. 
 
 "This shows the utterly barbaric nature of this act and the dangers that hundreds of innocent citizens in Salisbury could have faced," May said. 
 
 Johnson said Wednesday that the Kremlin underestimated the global response to the poisoning. 
 
 "If they thought that the world had become so hardened and so cynical as not to care about the use of chemical weapons in a place like Salisbury," he said, "if they believed that no one would give a fig about the suffering of Sergei and Yulia Skripal, or that we would be indifferent to the reckless and contemptuous disregard for public safety ... then this is their answer." ||||| A policeman stands outside the Zizzi restaurant in Salisbury, England Wednesday, March 7, 2018 near to where former Russian double agent Sergei Skripal was found critically ill. Britain's counterterrorism... (Associated Press) 
 
 LONDON (AP) — A Russian ex-spy and his daughter fighting for their lives in an English hospital were attacked with a nerve agent in a targeted murder attempt, British police said Wednesday. 
 
 The case has further strained relations between Russia and Britain, which has said it will respond strongly if the Russian government is linked to the attack. It has overtones of a 2006 fatal attack on a former Russian spy that was blamed on the Kremlin. 
 
 In that incident, a radioactive poison was used. The choice of a nerve agent in the latest case follows the use of the banned nerve agent VX to kill the estranged half-brother of North Korea's leader last year. 
 
 Sergei Skripal, 66, and his 33-year-old daughter Yulia were found unconscious on a bench in the southwestern English city of Salisbury on Sunday, triggering a police investigation led by counterterrorism detectives. Baffled police initially said the pair had come into contact with an unknown substance. 
 
 "Having established that a nerve agent is the cause of the symptoms leading us to treat this as attempted murder, I can also confirm that we believe that the two people who became unwell were targeted specifically," Metropolitan Police counterterrorism chief Mark Rowley said. 
 
 Police said the two "remain in a critical condition in intensive care after being exposed to the substance." 
 
 Police have declined to speculate on who might be behind the attack. The Russian government has denied any involvement in the attack on Skripal, a former Russian agent who had served jail time in his homeland for spying for Britain before being freed in a spy swap. 
 
 Rowley said a police officer who treated Skripal and his daughter at the scene was in serious condition. He did not provide the officer's name or specifics about his condition. 
 
 Rowley didn't say what nerve agent was suspected in the attack. Nerve agents are chemicals that disrupt the messages sent by from the nerves to the body's organs. They can be administered in gas or liquid form, causing symptoms including vomiting, breathlessness, paralysis and often death. Officials have not offered a prognosis for Skripal and his daughter. 
 
 Sally Davies, the chief medical officer for England, said there was a low risk to the public. 
 
 Police and forensics officers continued to scour several sites in and around Salisbury on Wednesday. Police kept residents away from an Italian restaurant and a pub in the city, and cordoned off part of a business park about nine miles (14 kilometers) away, near the ancient stone monument of Stonehenge. Detectives appealed for information from anyone who visited either the Zizzi restaurant or the Bishop's Mill pub in Salisbury on Sunday. 
 
 Residents saw their usually placid town, famed for its 13th-century Gothic cathedral, turned into the center of a criminal probe with Cold War echoes. 
 
 With nerves still on edge, ambulances and emergency vehicles rushed to a building beside the Zizzi restaurant, which remains cordoned off. Witness Toni Walker said emergency services escorted two women from the building. Police and ambulance services declined to comment and it wasn't immediately clear if the incident had anything to do with the ongoing investigation. 
 
 Home Secretary Amber Rudd chaired a meeting of the government's emergency committee, known as Cobra, to discuss the investigation, which is now in the hands of counterterrorism police. 
 
 "We need to keep a cool head and make sure we collect all the evidence we can," Rudd said. "And then we need to decide what action to take." 
 
 Moscow officials, angered by allegations of Russian state involvement, accused Britain of using the case to fuel an "anti-Russian campaign" and further damage ties with Britain. 
 
 "What happened to Skripal has been immediately used to further incite an anti-Russian campaign in Western media," said Russian Foreign Ministry spokeswoman Maria Zakharova. 
 
 Skripal, a former colonel in Russia's GRU military intelligence service, was convicted in 2006 of spying for Britain and imprisoned. He was freed in 2010 as part of a widely publicized spy swap in which the U.S. agreed to hand over 10 members of a Russian sleeper cell found operating in America in return for four Russians convicted of spying for the West. 
 
 He and his daughter were found collapsed on a bench near a shopping mall Sunday in Salisbury, 90 miles (145 kilometers) southwest of London. 
 
 Foreign Secretary Boris Johnson told lawmakers Tuesday that if Moscow is shown to have been involved, the government would act — possibly downgrading England's participation in this year's soccer World Cup in Russia. 
 
 While police say they are keeping an open mind about the case, it has reminded many in Britain of the 2006 poisoning of former spy Alexander Litvinenko. 
 
 A British inquiry into his death found that Russian agents poisoned him by lacing his tea with radioactive polonium-210 and that the killing was probably approved by Russian President Vladimir Putin. Russia has denied any involvement in the Litvinenko case, and refused a request by Britain to extradite his accused killers. 
 
 Litvinenko's widow, Marina, wrote Wednesday in the Times of London that her husband's killing made clear to Britain's emergency services that they need to act quickly when "someone suddenly falls mysteriously ill." 
 
 "I am happy my story has raised awareness about the potential danger posed by Moscow, and this could help to save somebody's life," she wrote in an opinion piece. 
 
 ___ 
 
 Associated Press writer Gregory Katz in London contributed to this report. ||||| Image copyright EPA/ Yulia Skripal/Facebook Image caption Sergei Skripal, 66, and his daughter Yulia, 33 were poisoned by a nerve agent called Novichok 
 
 Yulia Skripal, the daughter of ex-spy Sergei Skripal, is improving rapidly and no longer in a critical condition, says the hospital treating her. 
 
 She and her father were admitted nearly four weeks ago after being exposed to a nerve agent in Salisbury. 
 
 The BBC understands from separate sources that Ms Skripal is conscious and talking. 
 
 However Mr Skripal remains in a critical but stable condition, Salisbury District Hospital said. 
 
 Doctors said Ms Skripal, "has responded well to treatment but continues to receive expert clinical care 24 hours a day". 
 
 "I want to take this opportunity to once again thank the staff of Salisbury District Hospital for delivering such high quality care to these patients over the last few weeks," said Dr Christine Blanshard, Medical Director for Salisbury District Hospital. 
 
 "I am very proud both of our front-line staff and all those who support them." 
 
 BBC correspondent Duncan Kennedy says a corner appears to have been turned for Yulia Skripal, who is now in a stable condition. 
 
 However there is no news on what the long-term damage might be, he adds. 
 
 Meanwhile the police have placed cordons round a children's play area at Montgomery Gardens near Mr Skripal's home. 
 
 "Officers will be searching it as a precautionary measure," said Deputy Assistant Commissioner Dean Haydon. 
 
 The Skripals were admitted to hospital on 4 March after being found collapsed on a bench at the Maltings shopping centre in Salisbury. 
 
 Police have been treating the case as attempted murder. 
 
 On Wednesday, police said the Skripals first came into contact with the nerve agent at the former Russian spy's home in Salisbury. 
 
 Image copyright EPA Image caption Police stand outside the house believed to be the home of Sergei Skripal 
 
 Forensic tests show the highest concentration was found on the front door. 
 
 The nerve agent was found at other locations in the town but in lower concentrations, the Metropolitan Police said. 
 
 A police officer who fell seriously ill after responding to the attack - Det Sgt Nick Bailey - was treated in hospital but was discharged on 22 March. 
 
 DS Bailey, who is believed to have visited Mr Skripal's house after the incident has spoken of his ordeal, saying: "Normal life for me will probably never be the same." 
 
 The British government has accused the Russian state of involvement in the attack - a claim Moscow has denied. 
 
 Prime Minister Theresa May said the chemical used had been identified as being part of a group of nerve agents developed by Russia known as Novichok. 
 
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 In response to the incident, Mrs May announced a series of sanctions including the expulsion of 23 Russian diplomats. 
 
 As a counter-measure the Kremlin said it would expel an equal number of British diplomats from Russia and close down the country's British Council. 
 
 Then - in what is believed to be the largest collective expulsion of Russian intelligence officers in history - more than 20 governments expelled diplomats in their countries, including the US, which removed 60. 
 
 In reply, Russia will expel 60 US diplomats and the US consulate general in St Petersburg is to be closed.
– The military-grade nerve agent that left former Russian double agent Sergei Skripal and his daughter near death appears to have been left on his front door in Salisbury, England, police say. Investigators say the Skripals first came into contact with the nerve agent at Skripal's home, and the highest concentration of it was found on the door, the Guardian reports. Skripal and his daughter, Yulia, were found unconscious on a bench at an outdoor shopping complex on March 4. Detective Sergeant Nick Bailey, who was hospitalized for more than two weeks after the incident, is believed to have become sick after visiting the Skripal house, where he was one of the first on the scene, reports the Telegraph. Police say residents of Skripal's neighborhood can expect more searches in the area, but "the risk remains low." Britain blames Russia for the attack. It has expelled 23 Russian diplomats and numerous other countries, including the US, have followed suit. In a speech late Wednesday, British Foreign Secretary Boris Johnson thanked Britain's allies "from the bottom of his heart," CNN reports. "I believe that these expulsions represent a moment when a feeling has suddenly crystallized, when years of vexation and provocation have worn collective patience to (the) breaking point, and when across the world—across three continents—there are countries who are willing to say enough is enough," he said. (A relative says there is almost no chance the Skripals will survive.)