Sinn Fein president Gerry Adams has denied sanctioning the murder of a British spy in the IRA.
The allegation about the 2006 killing of Denis Donaldson was made by a man who claimed he was also a paid state agent in the IRA.
The man made a series of claims, on an anonymous basis, to BBC Northern Ireland's Spotlight programme.
The most explosive was that Mr Adams sanctioned the killing of Mr Donaldson.
Mr Donaldson, 55, a Sinn Fein official and close colleague of Mr Adams, was shot dead at an isolated cottage near Glenties in Co Donegal in April 2006.
He had been living in the remote area of Donegal following his exposure as a state agent a year before his death.
Dissident republican group the Real IRA claimed responsibility for the murder in 2008 but the circumstances surrounding Mr Donaldson's outing as a British agent and subsequent death have long been shrouded in mystery.
The man interviewed by Spotlight said the IRA was responsible for the murder and Mr Adams sanctioned it.
The claim was rejected by the Sinn Fein leader, who has always denied he was in the IRA. His lawyer told the programme his client had no knowledge of or involvement in Mr Donaldson's death and denied being consulted about it.
Mr Adams's party colleague Gerry Kelly dismissed the documentary as a "collection of discredited conspiracy theories".
"The programme makers have had no regard for the feelings of families of the victims of the conflict, including the family of Denis Donaldson," said the North Belfast Assembly member.