Two men have been arrested by PSNI detectives investigating the murder of Eamon Collins in Newry in Co Down 15 years ago.
Mr Collins, 45, a former member of the IRA, was battered and stabbed to death while out walking his dogs in January 1999.
The two men were arrested this morning and taken to Antrim police station for questioning.
Mr Collins was attacked in the Dorans Hill area of Newry, a country road a short distance from his home.
Detective Chief Inspector Peter Montgomery, from the police's serious crime branch, said: "I would ask those with information about Mr Collins' murder, particularly those in the Barcroft estate, to think again about the awfulness of what happened and about the Collins family.
"It is never too late to come forward."
Mr Collins, a former IRA member who developed what was described as his own brand of "naming and shaming" of alleged republican activists, had been living in a staunchly republican area at the time of his death despite his criticism of the IRA.
While in the organisation, Mr Collins collected information on police officers and RUC special branch members and set up killings over a six-year period.
He was arrested in 1985 and charged with 50 terrorist offences, including five murders and membership of the IRA.
But he turned supergrass after his arrest and said he was prepared to give evidence against former colleagues.
Many suspects were arrested but most were released, after Mr Collins changed his mind.
A judge dismissed his alleged confessions and he was freed.
He later wrote a book, 'Killing Rage', which was highly critical of the IRA.
Police said in January of this year that they had DNA obtained from the murder scene.
