Police search the home of Sergei SkripalCredit:Paul Grover Sergei SkripalCredit:Social media; EAST2WEST NEWS Sergei Skripal with his daughter YuliaCredit:Social media; EAST2WEST NEWS As the cordon at Col Skripal’s home was extended to include the whole of his road, a large blue forensic tent was erected on the street and more police and incident support vehicles arrived from South Western Ambulance Service.A tent has also been erected in Col Skripal’s back garden while the ambulance station of the vehicle which took him to hospital, Zizzis restaurant and an office block next door have also been taken over as part of the investigation.Meanwhile, a photograph emerged of Col Skripal and his daughter raising their drinks in the same branch of Zizzi’s Italian restaurant in which they ate shortly before the attack. Mrs Skripal died of cancer in 2012 aged 59. Their son, who was 43, died whilst on holiday in St Petersburg last July, apparently from liver failure although it was been suggested that no record of his death can be found. He said: “It would have to have been been provided by a very specialised scientific facility. In other words, it was state sponsored.”Whoever it was handed to would have needed a high degree of expertise themselves. Sergei Skripal with his daughter YuliaCredit:Social media; EAST2WEST NEWS In other pictures, thought to have been taken relatively recently, the pair clink their glasses over dinner in a home and enjoy a meal in a restaurant. In one, Skripal poses with a beer in a pub. Police and forensic experts at the sceneCredit:Eddie Mulholland The investigation into the Russian spy attack intensified on Thursday as police sealed off a section of the cemetery where Sergei Skripal’s wife is buried.The cordon around the former double agent’s detached home in Salisbury was also extended as pressure mounted on the counter-terrorism officers leading the investigation to identify who was behind the sophisticated plot.The developments raise the prospect that detectives believe Col Skripal, 66, and his daughter Yulia, 33, may have been monitored in the days before they were attacked with a deadly nerve agent on Sunday.The involvement of hero police sergeant Nick Bailey, who remains in intensive care after rushing to the pair’s aid when they were found slumped on a shopping centre bench, has led to calls for the Government to take a hardline approach against Russia if state involvement is confirmed.Officers were yesterday guarding London road Cemetery in Salisbury, where Col Skripal’s wife, Liudmila, was buried in 2012 and where his son, Alexander, is commemorated with a plaque.Questions have been raised about the deaths of both following the assassination attempt. One of the theories being probed is that their food or drinks were spiked with the deadly nerve agent.Combing through the city centre CCTV has been one of the main lines of investigation, sources said as well as identifying anyone who had been in contact with the Skripals in the days before their attempted murder.Prof Andrea Sella, an inorganic chemistry expert at University College London also told the Telegraph that any residue from the nerve agent would provide possible clues.Hamish de Bretton-Gordon, a former commander of the British Army’s chemical, biological, radiological and nuclear regiment, said such an attack could only have been carried out by well trained operatives. It emerged that both Col Skripal and his daughter, Yulia, who are now both fighting for their lives in hospital, visited the cemetery last week, leaving flowers to mark what would have been Mr Skripal’s 44th birthday.A Wiltshire detective suggested the cordon had been extended around the house and erected around the graves in the hunt for forensic clues to the identities of the assassination squad.The source said: “The only reason to put up those cordons is to examine the area to see if the pair were under surveillance by hit squad. They will be looking in bushes and undergrowth for clues.” He said: “If this had been a shooting, we would be much further on than we are now. This is classic, clandestine, spy-type operations.”Will Geddes, who runs the International Corporate Protection agency, said the rare nerve agent used in the attack could be like VX – one of the deadliest chemicals known to man. “You can administer these things either as a spray or a liquid. A spray would be much more conspicuous, and instantaneous. For instance, if they had been sprayed in the eyes they would have collapsed almost immediately.”A liquid is better as it is more concentrated. There is also a delayed effect as it has to be ingested.”The Defence Science and Technology Laboratory in Porton Down, which has state-of-the-art equipment to look for trace amounts of substances, is believed to have been involved in examining the substance. Want the best of The Telegraph direct to your email and WhatsApp? Sign up to our free twice-daily Front Page newsletter and new audio briefings.