A new independent body for investigating Troubles murders in Northern Ireland could take two years to start work, the region's chief constable has said.

George Hamilton said he was anticipating an 18 to 24 month time-frame for the legislation required to create the proposed Historical Investigations Unit (HIU) to run its course.

Mr Hamilton vowed that, in the meantime, the Police Service of Northern Ireland's newly assembled Legacy Investigations Branch (LIB) would pursue the cases in a professional and determined manner and would not repeat the mistakes of its controversial predecessor, the Historical Enquiries Team (HET).

The creation of the HIU was agreed by Stormont's leaders and the British and Irish governments as part of December's Stormont House political deal, which resolved a series of disputes destabilising powersharing in Belfast.

The HIU will take on the criminal justice element of investigating the past while a separate truth recovery mechanism will endeavour to give bereaved relatives the chance to learn more about the circumstances of their loved ones' deaths without the prospect of conviction.

"It's probably 18 months to two years away before the creation of the HIU, in the meantime our statutory responsibilities don't change," said the chief constable.

The LIB started work at the start of the year after the axing of the HET.

The HET, which was set up in 2005 to investigate more than 3,000 murders, ultimately closed due to budgetary pressures, but before that it had already suffered significant reputational damage, with inspectors alleging it did not probe killings carried out by the military with the same rigour as those committed by paramilitaries.

Mr Hamilton pledged that the LIB would:

:: take on the 990 cases the HET had to yet to look at;

:: review all the military cases the HET had already examined;

:: look again at HET reviews that bereaved relatives were unhappy with;

:: prioritise those cases with the best evidential opportunities.

The HET, which was funded to the tune of £6 million a year, was envisaged as an arms-length unit outside the direct operational control of the PSNI.

Its 200-plus workforce was made up of agency workers, many retired police officers, and when the PSNI was forced to cut its agency contract due to budget pressures the HET was a direct casualty.

In its stead, the PSNI has taken responsibility for the HET's caseload into a newly formed and less well-resourced internal unit. The LIB currently has 50 officers, with 20 more due to start at the end of March.

"We think it is a smaller but much more professionalised and more agile department with greater accountability and transparency," said Mr Hamilton.

The chief constable rejected any suggestion the LIB would simply mark time until the HIU was set up.

"At the moment, with the pressure and push on resources and the reconfiguration we are needing to do just to try and maintain service delivery at current levels on normal everyday policing, I wouldn't be deploying 70 people to the Legacy Investigations Branch just to mark time," he insisted.

"It is a genuine effort based on professional judgment and volume of work to fulfil our legal responsibilities to investigate these things."

In 2013 HM Inspectorate of Constabulary claimed the HET's approach to investigations was inconsistent and had serious shortcomings.

The HMIC report was commissioned after the HET was criticised in an academic report that claimed the HET afforded former soldiers preferential treatment during interview and did not properly investigate deaths involving the Army.

"All the standards that HMIC said were lacking have now been fixed, we would argue," said Mr Hamilton.

"And we have invited HMIC back to do a health check against the recommendations - have we done what they recommended, both in the spirit and letter of it? We are pretty confident that's going to show we have."

At present the LIB is prioritising three major investigations -

:: the Bloody Sunday shootings in Londonderry in 1972;

:: the task of reassessing the evidence in the cases of around 200 individuals sent so-called on-the-run letters by the Government assuring them they weren't being sought by police;

:: allegations that a defunct Army unit, the Military Reaction Force, carried out indiscriminate shootings during the Troubles.

At least two of those investigations are predicted to run for many years. But Mr Hamilton insisted other HET cases would be taken on while work continued on the three current probes.

But he said he did not want to offer false hope about what could be achieved in the time-frame and with the current level of resources.

"I need to deal with people's expectations," he acknowledged.

Mr Hamilton said much of the detail around how the HIU would work had yet to be determined. Issues around who will staff the new unit and whether it will take possession of all files and evidence from the PSNI have not been hammered out.

The region's top officer said relinquishing the workload would help the PSNI concentrate more fully on policing the present.

But he insisted the real beneficiaries would be those in the victims' sector.

"I think it will be a relief for families and victims and survivors that actually there's now a process in place with a clearly identified budget and system set up to deal with and bring some degree of closure to their pain and their loss," he said.

Mr Hamilton added: "We are torn at the moment between policing the risk and crime and the communities today and what's happening today, with also recognising we have a legal responsibility to investigate the past.

"These aren't just cases sitting in box files, they are actually people with aching hearts and still grieving and still having unanswered questions so there is a very human side to this that we acknowledge and, for us, the creation of the HIU is an indication from the (Stormont) parties and the governments that actually this is going to be taken seriously, going to be dealt with professionally, the resources are going to be put to it and we welcome that."

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