Stakeknife: Detectives from outside Northern Ireland to investigate
A team of detectives from police forces outside Northern Ireland is expected to investigate the activities of the army's most high-ranking agent within the IRA.
The agent, codenamed Stakeknife, stands accused of involvement in up to 50 murders.
He has been named by the media as former west Belfast man Freddie Scappaticci.
He has denied the allegations.
The investigation is expected to be the largest ever in the UK into allegations against a single individual.
Sources have told the BBC the case could take a team of up to 50 detectives at least five years to complete.
It is understood Police Service of Northern Ireland (PSNI) Chief Constable George Hamilton is considering the option of using detectives from other police forces in an attempt to secure the support of relatives of Stakeknife's alleged victims, who have called for a fully independent investigation.
No single police force in England, Scotland or Wales is likely to be willing to release the number of detectives needed for the case.
The PSNI is expected to advertise for experienced detectives throughout Great Britain to apply to join a special major investigation team on secondment.
The team will report directly to Mr Hamilton, who will report to the Northern Ireland Policing Board about its activities.
Bringing in a team of experienced detectives from other UK police forces may be easier said than done.
The cost is the first problem, as an investigation of this scale and complexity will have a price tag of millions of pounds.
The chief constable will first of all have to secure the funding needed from the Department of Justice and possibly the government at Westminster.
Even if the money is available, the number of detectives needed may not be.
Police forces throughout the UK are facing significant budget cuts and may not have sufficient resources to allow officers to go on secondment.
They will investigate allegations about the activities of Stakeknife and the IRA's internal security unit, the so-called "nutting squad", between 1978 and 1995.
The unit was responsible for identifying, interrogating and killing IRA members suspected of being informers.
It has been claimed that Mr Scappaticci, the son of an Italian immigrant, was a leading member, and the army agent codenamed Stakeknife.
Last month, the director of Public Prosecutions in Northern Ireland, Barra McGrory QC, announced that he had asked the police to investigate allegations that Stakeknife was involved in 24 murders.
That number could grow significantly.
'Step in the right direction'
The daughter of a woman, who was allegedly murdered by the same unit, has welcomed the news, saying it is "a step in the right direction".
Belfast woman Caroline Moreland, a Catholic mother of three, was abducted and murdered by the IRA in July 1994.
The body of the 34-year-old was found near Rosslea, County Fermanagh.
Speaking on BBC NI's Good Morning Ulster on Tuesday, Shauna Moreland said: "It's welcome news, we want an independent investigation.
"We don't think the PSNI would investigate fairly. For it to be fair, it needs to be resourced from outside."
She said she was not disappointed at the anticipated five-year wait for the conclusions of the investigation.
"Five years is a long time but we knew it was a process - we knew we wouldn't get answers within a few months," she said.
The IRA's internal security unit is believed to have killed at least 53 people it claimed were informers between 1978 and 1995.
It is unlikely that Stakeknife had a role in of all of those killings, but they will all have to be re-investigated in an attempt to establish precisely the extent of his involvement.
Detectives will investigate the activities of all members of the IRA's internal security unit during that 17-year period to establish exactly how many people it killed, and the identities of all of those involved.
The police team will also investigate possible criminal behaviour by those within the Ministry of Defence, the army and the security service MI5 who worked with the agent, or received information he passed on.
It has been claimed that some of them may have directed the agent's activities, or failed to protect lives by acting on information he provided.
The Police Ombudsman will continue to investigate similar allegations about former Royal Ulster Constabulary Special Branch officers.
While other police forces may be willing to release detectives on secondment, they will not be prepared to pay the costs involved.
It is estimated that the cost of establishing a major investigation team of 20 detectives for a year is about £1m.
That means a team of 50 detectives for five years would cost £12.5m.
But the bill would be much higher if detectives are brought in from police forces in England, Scotland and Wales.
The cost of weekly flights in and out of Northern Ireland, accommodation and subsistence payments would also have to be met.
While the PSNI will not conduct the investigation, it will bear ultimate responsibility for the cost.
However, an investigation of this scale was not anticipated when the policing budget was allocated.
It is likely that the chief constable will ask the Department of Justice at Stormont for additional funding for this additional demand.
Northern Ireland Secretary Theresa Villiers could also be asked to secure funding from Westminster as Stakeknife was allegedly acting as an agent of the state during direct rule.
Mr Hamilton has said several times that his priority is for the PSNI to keep people safe today, and not tie up resources investigating the past.
Spotlight is on BBC One Northern Ireland at 22:40 GMT on Tuesday.