BBC NI Home Affairs Correspondent
Claims of collusion between members of the IRA and the security forces will be investigated by the Police Ombudsman.
Nuala O'Loan will initially look into six incidents over a 20-year period.
It is thought that a number of IRA killers were protected from prosecution because they were working as agents for Special Branch and other agencies.
Nuala O'Loan will initially look into six incidents
One line of inquiry will focus on Belfast man Freddie Scappaticci, who was unmasked almost four years ago as an IRA informer codenamed Stakeknife.
He was head of the IRA's notorious internal security unit, which interrogated and killed those it decided were informers.
Three IRA members were shot dead by the unit in July 1992, and it is claimed they were killed to protect another more high-ranking agent.
The scope of the investigation could be expanded if there are further credible allegations.
But Ms O'Loan said she was not yet labelling anyone as an informer.
"On the republican side, the allegations are that there was protection of republican criminals," she said.
"And in some cases the suggestion is that there was a republican informant involved."
Investigators have already been in touch with the police - and will soon be contacting the Army and intelligence agencies.
They will ask for information about some of the IRA agents alleged to have been working for them.
The family of a man murdered by the IRA want his killing to be included in Mrs O'Loan's investigation.
Anthony McKernan from the Markets area of Belfast was shot dead by the IRA in January 1988.
The IRA said he was an informer, an allegation that Mr McKernan's family has always denied.
They claim their father was one of those murdered to cover for Scappaticci.
Speaking on the Nolan Show, Mr McKernan's daughter, Sharon Murtagh, said they believed there was collusion in his killing. "It would have been in the British interest to take my father out because my father was a member of the IRA.
"He done jail, he was on the run, he was an alleged bomb maker although he was never charged with anything in connection with explosions.
"Our theory is that it would be in their interests to take him out. But we also feel that my father was under Freddie Scappaticci and that they both colluded together to take him out."
For a full report, see BBC NI Newsline at 1830 GMT.