Stakeknife: Actions of IRA members, agents, Army and police to be examined
An investigation into the alleged activities of the Army's most high-ranking agent in the IRA will also examine possible crimes by IRA members, agents, Army and police handlers.
The agent, codenamed Stakeknife, has been linked to more than 50 murders.
The investigation will be led by the chief constable of Bedfordshire Police.
The PSNI said it would "not seek to direct or control, or in any way interfere" with the inquiry, which could take up to five years.
The agent known as Stakeknife has been named in the media as Freddie Scappaticci, originally from west Belfast. Mr Scappaticci has denied the allegations.
During a press briefing on Friday, PSNI Chief Constable George Hamilton said the investigation team would not include any current or former members of the Royal Ulster Constabulary (RUC), PSNI, Ministry of Defence or MI5.
"After taking a number of issues into consideration, I have decided that a team resourced with external officers and staff funded by the PSNI is the most appropriate way forward, given the size, scale and complexity of the investigation," he said.
The chief constable said the team would consist of between 50 and 70 investigators.
Mr Hamilton also introduced the man who will lead the investigation, Jon Boutcher, the chief constable of Bedfordshire Police.
"I am humbled to have been asked to lead such a critically important and complex investigation," he said.
"I do not underestimate the huge task of establishing the circumstances behind how and why these murders occurred during those dark days.
"My principal aim in taking responsibility for this investigation is to bring those responsible for these awful crimes, in whatever capacity they were involved, to justice.
"As soon as officers and staff are in place the investigation team will begin reaching out to victims, victims' families and all interested parties to receive information."
Who is Stakeknife?
Freddie Scappaticci is alleged to have been the most high ranking British agent within the Provisional IRA who was given the codename 'Stakeknife'.
He was the grandson of an Italian immigrant who came to Northern Ireland in search of work.
He has admitted, in the past, to being a republican but denies claims that he was an IRA informer.
He is believed to have led the IRA's internal security unit, known as 'the nutting squad,' which was responsible for identifying and interrogating suspected informers.
Mr Scappaticci left Northern Ireland when identified by the media as Stakeknife, in 2003.
In October 2015, 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.
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.
The daughter of a woman who was allegedly murdered by the IRA said she was relieved that the investigation was "finally starting".
Belfast woman Caroline Moreland, a Catholic mother of three, was abducted and murdered in July 1994.
The body of the 34-year-old was found near Rosslea, County Fermanagh.
Her daughter, Shauna, said: "I'm still very cynical of getting the whole truth but I don't think I would be able to go on with this fight believing that I couldn't get aspects of the truth at least."