A specialist team set up to review unsolved murders committed during the Northern Ireland Troubles say their task may be almost impossible.
Dave Cox and Phillip James of the HET
The Historical Enquiries Team, which has a budget of £34m, is re-examining 3,268 killings between 1969 and the 1998 peace accord.
The team has another five years to complete its work, with a target of 40 cases a month.
However, investigators now say they will need more time for their work.
Retired Metropolitan Police Commander Dave Cox, who is heading the team, said there was a lot of work ahead.
"We have met a lot of families and have been able to answer individual concerns in some cases," he said.
"But in terms of us being able to say hand on heart that we've completely finished this process, I don't think we're there with any of the cases we've looked at yet."
The Police Ombudsman, Nuala O'Loan, said she agreed. The ombudman's office investigates cases where there is evidence or allegations of police involvement.
To date, 78 such cases have emerged, including some of the Troubles' most controversial incidents, such as the murder of Belfast solicitor Pat Finucane.
Ms O'Loan said it was "unrealistic" to deal with 30 years of history in the time allotted by the government.
"What we're trying to do effectively here is come to terms with everything that has happened in the past," she said.
"It's not realistic to think that we'll do it in six years, that we will re-investigate all those murders and deaths. It's just not realistic I think.
"I think that we're talking at least a decade, probably, to do all those cases, possibly more. I think that the historical enquiries team will have the same problem."
The Northern Ireland Office, which funds the initiative, would not be drawn on whether more resources would be made available.
The review team uses the latest forensic science and intelligence analysing technology. The team is comprised of about 100 detectives and support staff.
The team is operationally independent from the PSNI, but reports to the chief constable.
The Northern Ireland Secretary, Peter Hain, has said it is possible that people would serve jail terms as a result of the new murder investigations.
Northern Ireland's Chief Constable, Sir Hugh Orde, has said he hoped the review team would bring closure to many families.