Image copyright EPA/ Yulia Skripal/Facebook Image caption Mr Skripal, 66, and his daughter Yulia, 33, are in a critical condition in hospital 
 
 More is known about the substance used in the suspected poisoning of a former Russian double agent and his daughter, the home secretary has said. 
 
 Speaking after chairing a meeting of the government's emergency committee Cobra, Amber Rudd said the police would be sharing details later on Wednesday. 
 
 Sergei and Yulia Skripal were found unconscious in Salisbury on Sunday afternoon and remain critically ill. 
 
 Police want to speak to anyone who was in the city centre around that time. 
 
 They are particularly keen to speak to people who ate at Zizzi or drank in The Bishop's Mill pub. 
 
 Meanwhile, fire and ambulance crews have been called to an office next to the Zizzi restaurant in Salisbury, but police are refusing to say whether this is connected. 
 
 Mr Skripal, 66, and his 33-year-old daughter were found slumped on a bench outside the Maltings shopping centre. 
 
 Scientists at the UK's military research facility at Porton Down have been examining a substance thought to be behind their collapse. 
 
 Media playback is unsupported on your device Media caption Amber Rudd provides an update on the substance that left a Russian man critically ill 
 
 Referring to what she called "the terrible events" in Salisbury, Ms Rudd said: "We need to keep a cool head and collect all the evidence we can. 
 
 "We need to make sure that we respond not to rumour but to all the evidence they collect and then we will have to decide what action to take." 
 
 She said the police investigation was likely to be lengthy and it was important to make sure they had the space to continue their investigation. 
 
 Earlier, Assistant Commissioner Mark Rowley, head of Counter Terrorism Policing, appealed for witnesses, saying: "Did you see anything out of the ordinary? It may be that at the time, nothing appeared out of place or untoward but with what you now know, you remember something that might be of significance... 
 
 "The weather was poor that day so there were not as many people out and about. Every statement we can take is important." 
 
 Image copyright PA Image caption Police in protective equipment responded to the incident 
 
 Cordons have been erected in and around Salisbury. One, at a business park in Amesbury, a town about 10 miles away, has been removed. 
 
 Two police officers caught up in the suspected contamination were treated in hospital for minor symptoms - believed to be itchy eyes and wheezing - before they were given the all clear. 
 
 A third member of the emergency services remains in hospital. 
 
 Relatives of Colonel Skripal, who was convicted in Russia in 2006 of spying for Britain, have told the BBC Russian Service that he believed the country's special services might come after him at any time. 
 
 It has been said his only friends in the UK are in the British intelligence services, but he has also joined his local railway club and given lectures at military academies. 
 
 His wife, elder brother and son have all died in recent years. 
 
 Media playback is unsupported on your device Media caption BBC Rewind looks back at cases of high-profile Russians targeted on foreign soil 
 
 Ms Skripal lives in Moscow and has visited her father in the UK regularly, especially during the past two years. 
 
 Family and friends say she did move to the UK but missed Russia so returned to Moscow where she has worked for multinationals, including Nike and PepsiCo. 
 
 It is thought her latest visit to the UK may have been timed to coincide with her late brother's birthday. 
 
 Alexander Skripal died aged 43 last July in St Petersburg from liver failure in what relatives believe to have been suspicious circumstances. 
 
 He is buried in Salisbury close to his mother, Liudmila Skripal, who died from cancer in 2012. 
 
 The possibility of an unexplained substance being involved has drawn comparisons with the 2006 poisoning of Alexander Litvinenko - a public inquiry concluded the killing of the Russian dissident was probably carried out with the approval of the Russian President, Vladimir Putin. 
 
 Former MI5 officer Annie Machon pointed out that the two cases were very different, however. 
 
 Mr Litvinenko was a whistleblower, dissident and consultant for MI6 actively at the time of his death, she said, whereas Mr Skripal has already been caught, convicted and pardoned and allowed to freely find safe haven in the UK. 
 
 "There's no conceivable reason that I can see that the Russian state would have been targeting him at the moment," she told BBC's Breakfast. "That's why we need to think about what else he might have been involved in." 
 
 Media playback is unsupported on your device Media caption BBC Newsnight visits the graves of Liudmila Skripal and Alexander Skripal in Salisbury 
 
 Russian Foreign Ministry spokeswoman, Maria Zakharova, said: "These people have been used by the foreign media for an anti-Russian campaign. 
 
 "It's a traditional campaign. The tradition is to make things up. We can only see it as a provocation." 
 
 On Tuesday, Foreign Secretary Boris Johnson told MPs the UK would respond "robustly" to any evidence of Russian "state responsibility" in the case. 
 
 Russia has insisted it has "no information" about what could have led to the incident, and says it is open to co-operating with British police if requested. 
 
 The Russian embassy in London accused the media of creating "an impression of a planned operation by the Russian special services, which is completely untrue". 
 
 Who is Sergei Skripal? 
 
 Image caption Col Skripal, 66, had been living in Salisbury after being released by Russia in 2010 
 
 Colonel Skripal, a retired Russian military intelligence officer, was jailed for 13 years by Russia in 2006. 
 
 He was convicted of passing the identities of Russian intelligence agents working undercover in Europe to the UK's Secret Intelligence Service, MI6. 
 
 In July 2010, he was one of four prisoners released by Moscow in exchange for 10 Russian spies arrested by the FBI. 
 
 After a Cold War-style spy swap at Austria's Vienna airport, Col Skripal moved to Salisbury, where he kept a low profile for eight years. 
 
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 Sergei Skripal, a former colonel of Russia's GRU military intelligence service, looks on inside the defendants' cage as he attends a hearing at the Moscow military district court, Russia August 9, 2006. Picture taken August 9, 2006. Kommersant/Yuri Senatorov via REUTERS 
 
 SALISBURY (Reuters) - Russian former double agent Sergei Skripal and his daughter Yulia remain in a critical condition in intensive care in hospital, British police said on Wednesday as they appealed for witnesses to come forward with any information about the case. 
 
 Skripal, 66, and Yulia, 33, were found unconscious on Sunday on a bench outside a shopping center in the southern English city of Salisbury after being exposed to an unknown substance. 
 
 Police said they were keen to speak to anyone who visited two venues in the city where Skripal and his daughter were thought to have been, the Zizzi pizza restaurant and the Bishop’s Mill pub. 
 
 “The focus at this time is to establish what has caused these people to become critically ill,” said Mark Rowley, head of London’s counter-terrorism police who are in charge of the investigation. 
 
 Police cordons remained in place in several locations in Salisbury, with new cordons added near Solstice Park in the nearby town of Amesbury. 

 SALISBURY, England (Reuters) - A nerve agent was used to deliberately poison a former Russian double agent and his daughter, Britain’s top counter-terrorism officer said on Wednesday, in a case that threatens to further damage London’s ties with Moscow. 
 
 Sergei Skripal, once a colonel in Russia’s GRU military intelligence service, and his 33-year-old daughter, Yulia, were found slumped unconscious on a bench outside a shopping center in the southern English city of Salisbury on Sunday afternoon. 
 
 Both remain critically ill and a police officer who attended the scene is also in a serious condition in hospital. 
 
 “This is being treated as a major incident involving attempted murder by administration of a nerve agent,” London Assistant Commissioner Mark Rowley told reporters. “I can also confirm that we believe the two people originally who became unwell were targeted specifically.” 
 
 Rowley said government scientists had identified the specific nerve agent but he would not say what it was because it was part of the investigation. He also declined to give any details about how it was administered to Skripal, 66, and his daughter. 
 
 England’s chief medical officer said the incident posed a low risk to the wider public but anyone feeling unwell was advised to seek medical advice. 
 
 While Rowley would not say any more about the investigation, a U.S. security source, speaking on condition of anonymity, said the main line of police inquiry was that Russians may have used the substance against Skripal in revenge for his treachery. 
 
 Skripal betrayed dozens of Russian agents to British intelligence before his arrest by Russian authorities in 2004. 
 
 He was sentenced to 13 years in prison in 2006 after a secret trial and in 2010 was given refuge in Britain after being exchanged for Russian spies caught in the West as part of a Cold War-style spy swap at Vienna airport. 
 
 On Tuesday, British Foreign Secretary Boris Johnson said if Moscow were behind the incident then Britain could look again at sanctions and take other measures to punish Russia, which he cast as a “malign and disruptive” state. 
 
 Russia denied any involvement, scolded Johnson for “wild” comments and said anti-Russian hysteria was being whipped up intentionally to damage relations with London. 
 
 “It’s very hard not to assess this (speculation) as provocative black PR designed to complicate relations between our two countries,” Foreign Ministry spokeswoman Maria Zakharova told reporters in Moscow on Wednesday. 
 
 “NEED TO DETER RUSSIA” 
 
 Malcolm Sperrin, a professor at the Institute of Physics and Engineering in Medicine, said nerve agents could cause heart failure, respiratory arrest, twitching or spasms. 
 
 “I’m not aware of a nerve agent having been used in this way previously,” he said. 
 
 Britain has specifically drawn parallels with the 2006 murder of ex-KGB agent Alexander Litvinenko who was killed with the radioactive polonium-210 in London. 
 
 Police vehicles are parked outside a restaurant near to where former Russian inteligence officer Sergei Skripal, and his daughter Yulia were found unconscious after they had been exposed to an unknown substance, in Salisbury, Britain, March 7, 2018. REUTERS/Toby Melville 
 
 A previous British inquiry said Russian President Vladimir Putin probably approved the murder of Litvinenko, who died after drinking green tea laced with the rare and very potent radioactive isotope at London’s Millennium Hotel. 
 
 Russia denied involvement in the death of Litvinenko, which the British inquiry said had been hatched by the Federal Security Service (FSB), main successor to the Soviet-era KGB. 
 
 Litvinenko’s murder sent Britain’s ties with Russia to what was then a post-Cold War low. Relations suffered further from Russia’s annexation of Crimea and its military backing for Syrian President Bashar al-Assad. 
 
 Former British defense minister Michael Fallon called for a stronger response if it turned out that Russia had been involved in the Skripal affair. 
 
 “We’ve got to respond more effectively than we did last time over Litvinenko. Our response then clearly wasn’t strong enough,” Fallon told Reuters. “We need to deter Russia from believing they can get away with attacks like this on our streets if it’s proved.” 
 
 The British capital has been dubbed “Londongrad” due to the large quantities of Russian money which have poured in since the 1991 fall of the Soviet Union. It is the Western city of choice for many oligarchs from the former Soviet Union. 
 
 BIG OPERATION 
 
 Rowley said hundreds of detectives were now working on the Skripal case to build up a timeline of his movements over the last few days. 
 
 Officers have sealed off the area of Salisbury where Skripal was found as well as the Zizzi pizza restaurant where they dined and the Bishop’s Mill pub where they had a drink. They also set up cordons near Solstice Park, a business park, in the nearby town of Amesbury. 
 
 Slideshow (10 Images) 
 
 Skripal’s son, Alexander, died last year aged 43. British media have said his death occurred during a visit to St Petersburg, Putin’s home town. Skripal’s wife Liudmila died of cancer in 2012. 
 
 Both Alexander and Liudmila Skripal are buried at the London Road Cemetery in Salisbury. 
 
 The Skripal case has come shortly before Russia’s presidential election on March 18, which Putin is expected to win comfortably, extending his rule by a further six years. The former KGB officer has been president since 2000.
– As former Russian double agent Sergei Skripal and daughter Yulia remain in critical condition after being found slumped unconscious on a bench in Salisbury on Sunday, UK authorities are issuing an appeal. "The weather was poor that day so there were not as many people out and about," says the head of counter terrorism policing in asking anyone who did brave the weather—particularly those who went to the nearby Zizzi restaurant or Bishop's Mill pub, where Reuters reports the two were thought to have stopped—to come forward with any details they can recall from that afternoon. "Every statement we can take is important." For now, the two are being treated for "symptoms rather than causes," as scientists at Porton Down, the country's military research facility, are trying to identify an unknown substance the pair are suspected to have been exposed to, the BBC reports. Reuters suggests progress has been made on that front, and quotes the country's interior minister as saying, "We do know more about the substance and the police will be making a further statement this afternoon in order to share some of that."