New twist in Diana SAS mystery: Why are detectives examining this image discovered on the elusive Soldier N's laptop... of snipers from his unit aiming at cars from a bridge in the UK? Picture from Soldier N's laptop has been passed on to Metropolitan Police The picture was one of 90 images of Special Forces soldiers found He faces investigation after he was said to have stored secret documents In all likelihood men in picture were engaged in counter-terrorism training By Sean Rayment and Ian Gallagher An SAS sniper, lying on a bridge, points his long-range rifle towards a dual-carriageway and peers into his telescopic sight, as if poised to open fire. It makes for a startling image - all the more so since the picture was taken not in a conflict zone or even a training camp but in a public area in the Welsh countryside. What makes it more arresting is that the photograph was found on a computer belonging to the Special Forces marksman known as Soldier N, who is said to have told his wife that members of the SAS "arranged" the death of Princess Diana. The Mail on Sunday can reveal that it has now been passed to the Metropolitan Police, whose specialist crime and operations command is investigating the sensational, if improbable, assassination theory. Startling: The photograph of the marksmen found on Soldier N's computer Last photo: Diana, driver Henri Paul and bodyguard Trevor Rees in her car moments before it crashed in Paris The allegation first came to light during the second court martial of Sergeant Danny Nightingale, who was found guilty of illegally possessing a gun and ammunition. Since then it has attracted global press attention and fuelled conspiracy theories. It was outlined in a letter, written by the mother-in-law of Soldier N, who was a key witness for the prosecution. The picture was one of 90 images of Special Forces soldiers found on Soldier N's home computer. He faces a Ministry of Defence investigation after he was also said to have illegally stored secret SAS tactical documents, videos of operations in Afghanistan and emails to his then wife from Afghanistan identifying the location of SAS and Special Boat Service units, times and dates of operations, and tactics used to kill and capture insurgents. In all likelihood the men in the photograph taken on the bridge were engaged in a counter-terrorism training exercise, practising a procedure known as high speed vehicle interdiction. The tactic was developed to stop vehicles being driven by terrorists or suicide bombers travelling at speed. Tour of duty: Ninety images of Special Forces soldiers were found on Soldier N's home computer It is thought that the bridge and a section of road beneath it were closed at the time. The Mail on Sunday knows the location of the bridge but has agreed not to disclose it at the request of senior defence officials. Author and former SAS soldier Andy McNab said that although the exercise would have been "as realistic as possible," the sniper would not have used either live or blank ammunition. Even so, it is easy to see how the 2009 image, thought to have been taken by Soldier N, might be seized upon by those who believe Diana's death, along with her boyfriend Dodi Fayed, in a car crash in a Paris underpass in 1997 was murder, not an accident. Simon McKay, solicitor for Dodi's father Mohamed Al Fayed, said it not only "causes concern and anxiety by everyone affected by this case but also the public generally, who are entitled to answers not just how it came about, but also how it was photographed and the extent to which the military sanctioned it." The Ministry of Defence declined to discuss the picture last night. Princess Diana hours before she died Soldier N is said to have claimed that a former member of the elite regiment was in charge of an assassination squad which moved in on Dodi's driver Henri Paul, who also died in the crash, using a white car and a motorbike - before flashing a blinding light into his eyes. But reflecting the twisting nature of the case, this has now been denied by Soldier N himself. A source close to the inquiry told this newspaper that he and his girlfriend gave statements to police last month, and that Soldier N blamed his former wife for "trying to cause trouble." Scotland Yard said yesterday it was "not appropriate to give a running commentary on the progress of the investigation." Meanwhile Mr McKay, acting on behalf of Mr Al Fayed and Soldier N's wife, has been critical of the Met's approach to the case. He wrote to the Met Commissioner, Sir Bernard Hogan-Howe, to complain that the officer leading the investigation, Detective Chief Inspector Philip Easton, was unlikely to be "sufficiently objective or open-minded." This, he said, was because DCI Easton was a "significant contributor" to the Paget Report, which concluded Diana's death was a tragic accident. Mr McKay said last night: "It is important to bear in mind that it is not disputed that Mr Al Fayed's son, Dodi, was unlawfully killed and that he is entitled to the same treatment that any father facing such a tragedy expects from the police in this country. The reality is the police have approached this new material with scepticism before exploring its truth. They have issued press releases without first speaking to the family. They have failed to meet promises that Mr Al Fayed would be kept up to date with inquiries. All of this fails to meet the basic requirements of their own victim support policy and minimum legal standards. There is now an incurable lack of confidence in how the Met have approached the matter and it should be dealt with by an independent police force. It is important to bear in mind that it is not disputed that Mr Al Fayed's son, Dodi, was unlawfully killed and that he is entitled to the same treatment that any father facing such a tragedy expects from the police in this country' Simon McKay, solicitor for Dodi's father Mohamed Al Fayed Scotland Yard insisted its officers are "looking for new evidence that is credible and relevant." A spokesman added: "The officers doing the assessment are a combination of those with a detailed knowledge and those not previously involved. Their work is being overseen by Deputy Assistant Commissioner Martin Hewitt who was not previously involved. Other documents said to have been stored on Soldier N's computer include files containing classified information revealing covert operations in which senior members of Al Qaeda and the Taliban were killed and captured. Soldier N also sent a series of emails to his then wife from Afghanistan identifying the location of SAS and Special Boat Service units, times and dates of operations and the tactics used to kill and capture insurgents. Defence sources last night described the security breach as a huge embarrassment to the SAS, which prides itself on secrecy and professionalism. It is forbidden for members of the SAS to keep highly sensitive information on personal computers and those doing so face charges under the Official Secrets Act. A security breach on such a large scale is understood to be unique within the SAS, and a Ministry of Defence investigation is trying to establish the extent of the problem. One source said: "The cardinal rule is never to talk about operations to anyone outside the SAS. To send emails over the internet naming members of the SAS, troop locations and details of forthcoming operations potentially endangered the lives of dozens of his fellow soldiers. Had this been known at the time, this individual would have been thrown out of the regiment and probably court-martialled. Letter: The allegations of SAS involvement in Diana's death first emerged in a seven-page letter written in September 2011 by Soldier N's mother-in-law The MoD said in a statement: "The MoD takes any allegations of data loss or breaches of security extremely seriously and we will always take appropriate action when these are brought to our attention. While serving, all military personnel should uphold the high standards and values the UK Armed Forces insist upon. Soldier N is alleged to have made the claim about Princess Diana after Prince William visited the regiment's headquarters in 2008 to undertake a special driving course. The conversation took place at Soldier N's home in Hereford in 2008 when he and his former wife were still together. Princess Diana and Dodi al-Fayed wait at the rear service exit of the Ritz Hotel in Paris on August 31, 1997 When Soldier N's wife said how sorry she felt for William because he had lost his mother in such tragic circumstances, her husband is alleged to have said "it was the SAS who killed her." He reportedly claimed the Princess was killed by an SAS hit team which flashed a high-powered light into the face of chauffeur Henri Paul, who was driving Diana and Dodi through Paris on August 31, 1997. The couple's car crashed into a pillar of the Pont de l'Alma underpass. The allegations of SAS involvement in Diana's death first emerged in a seven-page letter written in September 2011 by Soldier N's mother-in-law. Copies of the letter were sent to the SAS's commanding officer and to Dyfed Powys Police. But the contents were only disclosed following the court martial of Sgt Danny Nightingale. The letter states that Soldier N made a series of violent threats against his wife and her family following the collapse of the couple's marriage. The reference to Diana appears on page seven when Soldier N's mother-in-law writes: "He [Soldier N] also told her [his wife] that it was the SAS who arranged Princess Diana's death and that has been covered up. So what chance do my daughter and I stand against his threats? The letter led to the arrest of Sergeant Nightingale and Soldier N after police found illegally held firearms and ammunition at a house they shared in Hereford. Soldier N admitted the offences and was sentenced to two years at the Military Corrective Training Centre in Colchester, Essex. Nightingale also admitted the charges and received an 18-month sentence. Following a public campaign he was freed and the conviction quashed. But at a fresh court martial in July, he was found guilty and sentenced to two years suspended for 12 months. Solicitor Simon McKay, acting on behalf of Mr Al Fayed (pictured) and Soldier N's wife, has been critical of the Met's approach to the case Nightingale was largely convicted on the sworn evidence of Soldier N. The claims concerning the SAS involvement in Diana's death are now part of a "scoping exercise" being conducted by Scotland Yard. Detectives have interviewed the estranged wife of Soldier N, who is understood to have given police a "detailed and compelling" account of the claims allegedly made by her husband. Found on his computer were presentations on a series of SAS tactics describing how troops enter enemy territory undetected. Other documents refer to the intelligence snipers should be able to glean by observing targets. One document refers to a technique called "Free Drop Air Despatch" which is described as "an extremely effective method of long distance insertion using CH-47 (Chinook helicopters) to insert small teams into hostile areas." Another document is entitled: "Intelligence Required From Snipers" and details everything a sniper should look for when assessing a target. There are documents revealing how snipers identify targets hidden inside buildings. Also on the computer were a series of videos shot in Afghanistan showing members of the SAS and SBS firing high-powered sniper rifles from a Chinook helicopter. The videos identify members of the SAS, their equipment and tactics. Other videos show SAS snipers practising on ranges believed to be in Britain. One source last night said: "Had this fallen into the wrong hands the damage done to the SAS would have been horrendous. The identity of members of the Special Forces is never meant to be disclosed. Tactics, techniques and procedures - the building blocks of every SAS mission - would have been compromised. 'A kid got shot, but the bad guy was sorted': How Soldier N broke rules by revealing sensitive details in crass emails In one email sent to his wife, Soldier N writes about an SAS mission in which a child was shot and an elderly woman hit by shrapnel. In other emails written during a tour of duty in Afghanistan in 2009 he describes - sometimes gleefully - how insurgents are killed. Elsewhere he mentions the names of colleagues and locations where the SAS and Special Boat Service are based. One email discloses that the SAS were killing insurgents with missiles that had been fired from drones. Investigation: Soldier N faces an MoD investigation after he was also said to have illegally stored secret SAS tactical documents Crucially, Soldier N also reveals the dates when he and the rest of his unit were returning to Britain on leave. In one message written in August 2009, Soldier N tells his wife: "We have arrived in Kandahar, the next part is to get to Camp Bastion but our plane broke so we're staying here tonight with the guys from Poole [the Special Boat Service]. Don't you worry about me. You know that I shoot first, ask questions later. In another, written in the same month, he says: "We had a good morning today. One kid got shot though and an old chick got fragged [injured by shrapnel] but the bad guy and his oppo [colleague] were both sorted 1st we tracked a bad bomber but couldn't get him but an hour later 2 more bastards took over from him and were up to badness so we keeeeelled them with a missile. They made like jam and spread themselves all over the place! tee hee. We are watching you and we have missiles!! A good start to the day. Three months later, he writes: "The place I'm in now is quite alright, a bit like a massive villa in the mountains, no greenery though, it's better than anywhere else I've been so far, the lads are all cool, the boss is well switched on but chilled, not like A Squadron boss who's a total ******. I was glad the job went well last night, especially as I was Squadron Sergeant Major for it and we got a big player and a financier, so all is well. Later that month Soldier N writes: "We were out last night, so that's probably why you couldn't sleep. It was a good job. We got the bad man we were after and a few others. One kid got shot though and an old chick got fragged [injured by shrapnel] but the bad guy and his oppo [colleague] were both sorted. Two less bastards. I'm off to Kandahar tonight for a planning meeting, then the guys will follow in a few days. I should be able to keep in comms [communication] there.