Detectives investigating the deaths of four young recruits at an army barracks have been reported to the Police Complaints Authority.
The four recruits' families do not believe they committed suicide
Surrey Police said two senior detectives on the case were now being investigated after a complaint by the family of one of the victims.
The force carried out a £1m, 15-month investigation into the deaths of the soldiers, who all died of gunshot wounds at the Surrey base between 1995 and 2002, but found no grounds for prosecution.
Their families have never accepted the Ministry of Defence (MoD) view that they committed suicide.
Des and Doreen James, whose daughter Private Cheryl James was one of those to die, claim they were deliberately misled over the involvement of MoD officers in the inquiry.
Mr James, from Llangollen, north Wales, said they had initially been told the MoD police's role was advisory.
However, they had since discovered the two MoD officers were closely involved in the investigation and had interviewed witnesses, throwing serious doubt on claims of impartiality.
He said: "It's not a question of whether the MoD officers were good at their job, bad at it, or even if they were impartial - it's because we were not told at all they were involved with the investigation.
A Surrey Police spokesman confirmed: "Following a complaint received
from the James family concerning a dispute between them and Surrey Police about
whether they were informed about the presence of two MoD police officers in the
Surrey inquiry Thames Valley Police have been called in to fully investigate
this matter and it has been referred to the PCA as a matter of course."
Surrey's handling of the case has already been passed to Devon and Cornwall
Police to look at, following a number of complaints by the victim's families
earlier this year, including the use of MoD officers.
The two officers referred to the PCA were involved in Operation Nodule,
investigating the deaths of Private Sean Benton, 20, from Hastings, East Sussex
and Private James.
A later police investigation, Operation Nickel, also examined
the deaths of Private Geoff Gray, 17, from Seaham, Durham and Private James
Collinson, 17, from Perth.
The MP for Perth, Annabelle Ewing, representing the family of Private James Collinson, who also died at Deepcut, said the use of MoD police was "very disturbing".
The soldiers who died at the barracks were Private Geoff Gray, 17, from Seaham, County Durham; Private Sean Benton, 20, from Hastings, East Sussex; Private James Collinson, 17, from Perth, Scotland and Private Cheryl James, 18, from Llangollen, north Wales.
We referred last week to the fact that the Police Complaints Authority has been asked to look into a complaint made by the parents of one of the soldiers who died at Deepcut Barracks, Private Cheryl James.
They have claimed that they were misled over the involvement of officers from the Ministry of Defence in the investigation.
Detective Chief Inspector Colin Sutton has asked us to make it clear that he is not one of the officers involved in the PCA enquiry and that he was never connected with the investigation into Private James' death.