A dissident group which has called itself the IRA has claimed responsibility for the murder of Tyrone prison officer David Black.
On Monday, the Belfast-based Irish News newspaper said it received a letter from a coalition of dissident groups calling themselves the IRA.
"An active service unit of the IRA executed prison guard David Black," the IRA statement said.
"While the IRA never takes this type of action lightly, the IRA has a responsibility to protect and defend Republican PoWs (Prisoners of War)."
There is an ongoing dispute in the prison, where 41 dissident republican prisoners are currently detained.
The nationalist faction which simply describes itself as the IRA, is understood to be made up of three anti-British splinter groups led by former Provisionals.
On November 1, 52-year-old Black was gunned down during an ambush along the M1 motorway as he drove to work at Maghaberry jail, near Lisburn, County Antrim. A father of two, Black had worked as a prison officer for three decades and was considering early retirement.
It was the first killing of a police officer in almost 20 years.
Four men have been arrested by police in connection with the murder, but were all later released without charge.