The IRA's most senior security force informer is to be investigated about at least 24 murders.
The army double agent was known as Stakeknife, a shadowy figure himself responsible for finding and killing those it believed passed information to the British security services during the Troubles. Freddie Scappaticci has strongly denied being the man behind the codename.
A police watchdog has passed information to prosecutors after examining the circumstances of murders attributed to Stakeknife's IRA "internal security team".
At the heart of victims' concerns is whether those deaths could have been prevented and whether collusion in murder penetrated to the top of the British Government.
The Director of Public Prosecutions (DPP) in Northern Ireland, Barra McGrory QC, asked police to investigate potential offences committed by Stakeknife.
He said: "I have outlined today extremely serious matters, perhaps the most significant in my time as DPP.
"I have not taken the steps to commence investigations lightly but, rather, consider they must be taken to ensure that public confidence can be maintained in the office of the DPP and in the wider criminal justice system."
He added a common link across a significant number of potential crimes, including murder, was the alleged involvement of Stakeknife.
"I confirm today that I have requested that the chief constable investigate a range of potential offences which relate to the alleged activities of an agent commonly known as Stakeknife."
Northern Ireland's Police Ombudsman is investigating the murders of alleged informers by the IRA and the potential role of Stakeknife. It passed information to the DPP, which resulted in today's announcement.
Former Met Police commissioner Lord Stevens led three government investigations into security force collusion.
Relatives of the victims have pressed for a fourth more comprehensive and independent probe or public inquiry.
Frank Mulhern, whose IRA member son Joe was discovered in 1993 in a ditch near the Irish border in Co Tyrone with his body riddled by bullets, said there needed to be an independent investigation by an international police force.
He added: "It will continue to be covered up until we expose it and put a stop to it."
He said he had been pursuing the matter for many years and still hoped to receive justice.
"If he (the killer) was not being protected he would be in jail now. Of course he is being protected, even a blind man can see that."
Mr McGrory requested two separate investigations.
"The first will be an investigation of broad scope. This will seek to examine the full range of potential offences that may have been committed by Stakeknife.
"It will also include an investigation into any potential criminal activity that may have been carried out by security service agents."
PSNI ACC Will Kerr said police had received a referral from the Director of Public Prosecutions which the service was addressing.
"It would be inappropriate to comment further," he added.
Mr McGrory said Northern Ireland's attorney general John Larkin QC had recently been in contact with his office asking what action prosecutors may take about a particular murder implicating Stakeknife.
"I have identified one case where I consider that there is now sufficient information available at this point to review a prosecutorial decision. This relates to a case involving an allegation of perjury in 2003.
"I have serious concerns in relation to this decision. Having reviewed all the available evidence I consider that the original decision did not take into account relevant considerations and also took into account irrelevant factors.
"I have concluded that the original decision was not within the range of decisions that could reasonably be taken in the circumstances."
The decision has been set aside and the DPP asked the chief constable to provide further material.