The review of Surrey Police's reinvestigation of the deaths was conducted by Devon and Cornwall Police.
Privates Sean Benton, Cheryl James, Geoff Gray and James Collinson were each found dead from gunshot wounds at Deepcut, in Surrey, between 1995 and 2002.
The more recent deaths, in 2001 and 2002 - those of Ptes Gray and Collinson - gave rise to a major reinvestigation amid concern about what was going on at the camp.
Devon and Cornwall Police's review of Surrey Police's subsequent reinvestigation said officers had narrowed the focus of the inquiry, concentrating too much on suicide as the likely outcome.
The report said: "This mindset may have limited their focus and their use of some principles contained within the Murder Investigation Manual, which are designed to aid the gathering of information and evidence."
The report cited the multi-layered complexity of the inquiry - something that has frustrated bereaved relatives all along and led to calls for a full public inquiry.
"Further confusion occurred due to the differing layers of strategic involvement adopted by chief officers who did not have an auditable decision-making process in place," the report said.
Describing this as "a lack of clarity", the report criticised the Surrey force inquiry's leadership.
"The review also found it difficult to identify exactly who was in charge of the reinvestigations with SIOs, heads of crime and Acpo (Association of Chief Police Officers) all involved."
The report also noted how Geoff Gray, whose son Geoff died, obtained documents under the Freedom of Information Act from the MoD.
The review board from Devon and Cornwall Police said these "issues raised (by the documents) went to the heart" of the question of Surrey Police's mindset, the report said.
It was indicated that there was a desire to carry out further work on this but Surrey Police opposed it, the report said.
"Surrey Police subsequently decided no work should be undertaken by the Devon and Cornwall Constabulary review into these new documents. As a result, the findings within this report do not take into account these unresolved issues."
At a press conference at Surrey Police headquaters, Deputy Chief Constable Brian Moore responded to the report's criticisms.
He said Surrey Police did not allow the document Mr Gray unearthed from the army in April to be scrutinised by Devon and Cornwall in order to avoid charges of "one force helping out another".
The matter is currently in the hands of the Independent Police Complaints Commission (IPCC), he said.
He said he did not accept that, because his officers treated the Deepcut deaths as "different", this meant they had an "impartial mindset."
He said "it was necessary to be different" when investigating four deaths of soldiers.
Mr Moore accepted that "certain principles" from the Murder Investigation Manual were not applied by his officers.
But he added: "They chose to apply different principles from the same manual."
Of the criticisms of the leadership of Surrey's inquiry, Mr Moore said: "I accept I did not provide Devon and Cornwall with sufficient auditable documents of the command structure."
He added that until any further evidence comes to light, via subsequent inquiries or hearings, he and his officers would not be further investigating the four deaths. "We welcome the findings," he added.
Mr Gray said of the review: "Surrey Police are trying to play down all of this but the moment they say there is a problem with the mindset, that sparks concern in my mind.
"If they haven't followed national guidelines, if they tried to put out stories in the media, it all stinks again.
"This just backs up the document we found in Geoff's army record saying that the end result would be suicide. The whole million-pound investigation has been shown to be a farce."
Devon and Cornwall Police's review of Surrey's reinvestigation ran from September 2003 until August this year.
The document published today was the "executive summary" rather than the full report.
David Petch, the Independent Police Complaints Commissioner for Surrey, said: "Our independent investigation into the complaint from Mr and Mrs Geoff Gray is making good progress.
"We have a copy of Devon and Cornwall Constabulary's report, which will assist us as we carry out our inquiry.
"We will be meeting senior officers in both Devon and Cornwall Constabulary and Surrey Police to discuss the report."
Last June the IPCC announced that it would look into the Grays' complaint about the Surrey Police investigation and said the events at Deepcut should be handled by its own investigators.Reuse content