The specialist police unit re-examining 3,268 murders during the Troubles now has "police documentation" in more than 90% of cases.
There was an outcry in September when it was revealed the Historical Enquiries Team (HET) had been unable to locate almost a third of the RUC files, relating to some of the north's most high-profile killings.
Up to 1,000 files were missing at that time.
However, the HET said a painstaking trawl of police stations by a dedicated unit has now turned up documentation on more than 90% of cases.
The 33-strong team worked for more than 18 months searching every building owned by the PSNI, at a cost of £500,000.
"They searched every roofspace and every cupboard," HET chief Dave Cox said.
Police headquarters at Knock in Belfast is the only place still to be searched.
Mr Cox, a former commander of the Metropolitan Police who served on the Stevens Inquiry into the murder of the Belfast solicitor Pat Finucane, acknowledges that not all of those files now in possession of HET are complete.
Remaining files and other vital evidence from retrieved files may have been destroyed during bombings of police stations and the forensic science laboratory.
However, there have also been suggestions that files have gone missing as a result of a "culture of concealment and cover-up" within the RUC.
The HET has begun investigations on 320 cases since it was officially launched in January.
It has just six years to complete its re-investigation of all 3,268 murders – which relate to 2,516 separate incidents.
This has led the team to draw up a 16-week working model for each case – although it has accepted that some may take longer to complete.
"Some are working through that process, some are ready for resolution and others are taking longer," Mr Cox said.
There will be 40 cases dealt with each month for the first two years and 65 a month for the remaining four years.
The team are meanwhile holding their ground amid attempts to spread money originally allocated solely for their project to other agencies.
Their £32 million budget was originally ring-fenced but now has to be bid for each year.
Mr Cox is sanguine about the development.
"I have a budget worked out for the six years that we are here.
"At this stage I have enough money to pay for the staff that I have got," he said.
"Our perspective is that we can't start a project like this and bow out half-way through.
"But in fairness to the government, no-one knows how much something like this costs because it has never been done before – £32 million was a best guess.
"It would be for us to get on with the task and try to budget with the budget given to us."
It has been pointed out by observers, however, that the Saville Inquiry into Bloody Sunday has so far cost a total of £172 million, including £34 million on computers and IT systems alone.
Most of this year's HET work has been taken up with preparation for the beginning of its investigations proper, with more than 100,000 'marks' previously deemed unreadable, but nonetheless saved by the RUC, loaded into an automatic fingerprint recognition system.
A dedicated team has also taken on the full-time task of simply tracing the families of those killed.
Of the 320 cases currently under investigation, 284 families have been contacted.
This does not necessarily include the number of families who have contacted the HET of their own volition.
However, the team are working through their cases chronologically – the only exceptions are those determined by international courts to be urgent and those where family members may themselves have only a short time to live.
An archive from newspaper and television libraries, books on the Troubles and court files has been created.
A new super-computer programme has also been specially designed for the massive task, which overlays data from the thousands of cases to allow officers to probe links between different murders across decades.
This includes 'gap analysis' which can show, for example, that when a certain person was in prison a certain type of murder stopped, and began again on their release.
However, Mr Cox said his team has found that often the answers families seek are simple ones that do not need sophisticated science.
"In many cases families will say that we're the first cops they have ever spoken to," he said.
"At first we didn't understand that, then we would go to retired officers and say 'Why didn't you go back' and there would be a number of very good reasons.
"For a start there was the sheer volume of cases they were dealing with – in one case 496 murders in one year.
"They were just swept away by the volume.
"Another factor was that it was quite difficult to maintain contact with families without putting them at risk.
"Where you had a murder on an estate, if a police car keep on turning up then people start to think 'What are they telling the police?'
"Then you have the fact that police are trying to keep as much information as possible to themselves so they didn't educate paramilitaries.
"They [paramilitaries] were very good at counter-intelligence and using it for the next job to make sure they didn't make any mistakes."
Despite this, Mr Cox acknowledges that the treatment of families during the Troubles did not meet the standards it should have.
"Overall families should have been kept better informed than they were.
"There is no doubt about that," he said.
"It has caused a lot of bitterness in some cases.
"For many of the families we are dealing with it is as real to them today as it was when it happened.
"They have grief and trauma and years of frustration and bitterness overlaid on top of it.
"Our family liaison officers have to give them quite a lot of support. The scale of that is massive."
Some of the questions asked by the families so far have been simple procedural ones.
"In some cases you even find they don't need to know who did it. They know who did it. They want to know, for example:
'Why was the body left lying for 24 hours?'.
"For others they feel that it is looking at pictures and seeing the body because they never saw that before.
"We have to be careful how we handle that."
Even in the era of 'modern policing', HET represents what Mr Cox describes as "a unique concept in world policing".
"Is it possible? Well it's at the top end of difficult," he said.
"But until we try it we don't know.
"If you get put off by the scale and complexity of a task and no-one picks up the ball then you're never going to get it started at all.
"If you look at a ship being launched, it goes down the slipway and seems to go underwater and then it comes up and floats.
"It is a difficult undertaking that we are attempting.
"For everybody to be moving 30-40 cases through is difficult but it's not impossible.
"It's been very humbling really.
"You are meeting these lovely people, sometimes quite elderly, and they are saying 'Sorry to bother you, can I just ask about the death of my son'.
"And you just feel 'Of course you're not bothering us. That's what we're here for'.
"It's a step beyond policing.
"What we are doing is putting resources into answering questions that aren't policing questions because that's what the family want.
"It is absolutely fascinating and very moving. It is probably the most difficult and challenging job that I've had in 33 years being connected with policing.
"It is the families that drive it and the families that make it difficult, because you listen to so many sad stories but they are the reward because if you can do anything for them it makes it all worthwhile."
