Allegations of security force collusion in at least 24 murders in Northern Ireland by an army intelligence agent known as Stakeknife should be investigated by police, the Public Prosecution Service (PPS) said.
Stakeknife was the codename for a high-ranking army agent within the IRA.
Some victims believe double agents within the republican organisation were permitted by the security forces to commit crimes including murder to gain the trust of gunmen.
Director of Public Prosecutions Barra McGrory QC said: "I can confirm today that I have requested that the chief constable (of the Police Service of Northern Ireland) investigate a range of potential offences which relate to the alleged activities of an agent commonly known as Stakeknife."
Two investigations will centre on Stakeknife. Former senior IRA member Freddie Scappaticci has always denied being Stakeknife, an agent of military intelligence.
In April it was revealed that Northern Ireland's Police Ombudsman, which investigates complaints against the police, was probing whether the killings, thought to date back to the 1980s, could have been prevented. The Ombudsman has now contacted prosecutors.
It is understood criminal reviews of the matters stopped some time ago.
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 this individual.
"It will also include an investigation into any potential criminal activity that may have been carried out by Security Service agents."
He added: "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."