Two former detectives arrested over the police handling of the inquiry into an Ulster Volunteer Force (UVF) murder have been released without charge.
Wednesday's arrests related to the case of Raymond McCord Junior who was murdered near Belfast in 1997.
One of the officers arrested was former CID detective Trevor McIlwrath.
Police Ombudsman investigators questioned them over allegations of perverting the course of justice and misconduct in public office.
Two premises were also searched by the investigators, the Police Ombudsman's office has said.
Mr McCord, 22, was beaten to death by members of the UVF and his body left at a quarry.
It has been claimed that at least one of those responsible for the McCord murder was working for police Special Branch at the time and this led to a major investigation by Police Ombudsman Nuala O'Loan.
It is understood the investigation, which has been going on for several years, has mushroomed into a wider inquiry involving police intelligence-gathering methods and the use of informers.
An interim report has already been submitted to the Public Prosecution Service and a report is expected to be published next month.
The Police Ombudsman's Office said the two former officers were being questioned by its investigators at separate police stations.
Police Ombudsman senior director of investigations, Justin Felice, described the operation as "unusual but absolutely necessary".
The former police officers were arrested shortly after 0700 BST with PSNI officers in attendance.
They were released on Wednesday afternoon.
The ombudsman's investigation also includes the UVF murder of Sharon McKenna in north Belfast in 1992.
The 27-year-old Newtownabbey taxi driver was shot in the sectarian attack at the home of an elderly friend whose dinner she was making.
Mr Felice said the complexity of the inquiry had meant examining how detectives carried out the hunt for Ms McKenna's killers.
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