Exclusive: As police investigate claims that the British military may have been involved in the death of Princess Diana, Channel 4 News has seen the letter in question.
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The letter, written in 2011 by the parents-in-law of an unidentified SAS soldier known only as Soldier N, includes a claim that the SAS may have been involved with the death of Princess Diana.
Channel 4 News has seen the letter in connection with the second court martial of another SAS soldier, sniper Sgt Danny Nightingale who was found guilty of illegally possessing a gun and ammunition.
The focus of the letter is about the man who went on to be Sgt Danny Nightingale's housemate and who was also one of the Service Prosecuting Authority's (SPA) key witnesses at the Nightingale court martial, Soldier N.
It was the XXX who arranged Princess Diana’s death and that has been covered up. Extract from letter about Soldier N
It is understood that during the trial the letter was handed to the SPA and sections which mentioned names or identifying details were redacted before it was released to the court.
In the letter, the parents-in-law of Soldier N begged the commanding officer of the SAS to intervene as Soldier N was allegedly threatening to kill them and intimidate their daughter.
The parents-in-law also wrote about the circumstances surrounding Princess Diana's death, saying: "He also told her [the daughter] that it was the XXX who arranged Princess Diana's death and that has been covered up.
"So what chance do my daughter and I stand against his threats?"
A box for 'private jobs'
The author of the letter also writes how Soldier N had threatened their daughter saying he could make her "disappear" and warned her of the existence of a box that some of the members of the SAS used for "private jobs".
The letter states: "They put in the box the name, address and details of what they want done and then one of them who wants to earn extra money takes the details out of the box and does that job."
They then ask the commanding officer if the names and contact details of their family are in the box.
The seven-page hand-written letter also makes reference to another member of the SAS who allegedly shot his wife and family. It states: "He reminded my daughter of a man... who had matrimonial trouble and he went home and shot her and the family."
It alleges that Soldier N had admitted to the family that he had killed women and children during his time in the SAS: "He insists on telling us about his killing escapades whilst working in his job. How he has killed women and children and a priest, whose big toe kept on wiggling although he was dead."
Soldier N recently completed a two-year term in military prison after admitting to keeping a Glock pistol, a grenade and ammunition at his home. The letter references numerous occasions when he had military equipment at his home.
"He has been frightening my daughter by bringing guns into the home. Laying them out in a row on the lounge carpet and saying 'let's invite friends round 'because he wants to show people. She found a gun in the understairs cupboard. He had a gun in his car and in his bedside cabinet... we have been told by him that he is allowed to do this because of the job he does."
The letter has now been handed to the Metropolitan Police.
The allegations, if true, ask some serious questions of Soldier N's mental state of health. He allegedly hid from a motorbike which was driving down the road because "he thought someone was after him".
The parents-in-law said: "We have said for sometime that this man is not presenting with normal behaviour and yet we were told he is fine. No doubt he is good at his job but he can't switch off and he brings the job home and treats us all very badly. There are times when we have been petrified."
They then request the help of the army, stating: "You have trained him and as such I feel that the army should step in and take some responsibility here."
Scotland Yard said: "The Metropolitan Police Service is scoping information that has recently been received in relation to the deaths and assessing its relevance and credibility.
"The assessment will be carried out by officers from the specialist crime and operations command. This is not a re-investigation and does not come under Operation Paget."
The Ministry of Defence said it was not commenting and it was now a matter for civilian authorities.
A 2008 Inquest jury returned an unlawful killing verdict on the death of Princess Diana and Dodi al Fayed.
Channel 4 News does not know the identity of Soldier N and we cannot independently verify of the allegations of his behaviour.
- Diana: The Witness in the Tunnel channel4.com
- Scotland Yard examines new information on Diana The Telegraph
- Mohamed Al Fayed trusts police to investigate with vigour The Mirror