The Police Ombudsman's office is to re-examine the handling of the so-called "shoot-to-kill" case in which the RUC shot dead three IRA men in Co Armagh in 1982, prompting a political split in Northern Ireland.
The file has been referred to ombudsman Nuala O'Loan after the Council of Europe, which considered a series of disputed shootings involving state forces, found that the case was not handled properly.
It believes the investigation was not compliant with European Court of Human Rights standards and pressed the British government to put matters right. It referred the case to Mrs O'Loan.
Her office said last night it would "take a couple of months at least" to go through the files on the case. Former deputy chief constable of Greater Manchester Police John Stalker investigated the controversy but his report into the controversial killings was never published.
Mrs O'Loan leaves her post in November to be replaced by former senior Canadian police officer Al Hutchinson who has been overseeing the roll-out of the Patten reforms on policing in Northern Ireland. The next steps in the investigation will almost certainly fall to him, the ombudsman's office said.
The three IRA members, Seán Burns and Eugene Toman (both 21) and Gervaise McKerr (33) were shot dead at a roundabout near Lurgan in November 1982. Their car was struck by 109 bullets. The three were unarmed at the time and two were suspected of links to the murder of three police officers two weeks earlier. Three RUC men were charged with murder but acquitted.
The Council of Europe said last month, after a meeting of the Committee of Ministers, that it believed an effective police investigation had not been completed in six cases including the "shoot-to-kill" incidents and the murder of Pat Finucane.
The ministers "recalled the continuing obligation of the United Kingdom to conduct effective investigations inasmuch as procedural violations of Article 2 [ of the European Convention on Human Rights] were found by the court in these cases and urged the United Kingdom authorities to take, without further delay, all necessary steps in order to achieve concrete and visible progress".
Unionists responded angrily, with a former head of the Police Federation claiming no PSNI member would help in the investigation. Jimmy Spratt, now a DUP Assembly member, said: "It is yet another witch hunt by Nuala O'Loan involving the former RUC Special Branch. No police officer will co-operate with her."
DUP policing board member Peter Weir added: "Rank and file officers within the police believe that the ombudsman has a political agenda and therefore have a clear absence of trust in the outcomes of her investigations."
Lagan Valley MP Jeffrey Donaldson said: "There is no benefit to the community in Northern Ireland in doing this. The ombudsman's fixation with past cases is damaging the reputation of her office and is undermining the prospect of moving Northern Ireland towards a better future."
Sinn Féin welcomed the development, while a brother of one of the dead men said families had continued to be kept in the dark about developments in the case.
South Belfast Assembly member Alex Maskey, who joined the policing board earlier this year, claimed the circumstances surrounding the killing of the three IRA men were clear.
"Europe has already laid the finger of blame at the door of Number 10. Now we need to see the full publication of the Stalker report and the immediate publication for all other inquiries relating to the policy of shoot-to-kill."