The Northern Ireland Police Ombudsman has opened a new investigation into the murder of IRA informer Denis Donaldson.
The office of Dr Michael Maguire confirmed a new inquiry is under way into the police's handling of the case.
Mr Donaldson, 55, a senior Sinn Féin official and close colleague of party president Gerry Adams, was shot dead at an isolated cottage in Co Donegal in April 2006.
He had been living there since his exposure as an MI5 agent the previous year.
The Real IRA claimed responsibility for the murder three years later, but the circumstances surrounding Mr Donaldson's outing and subsequent murder have been shrouded in mystery.
Three years ago, the then ombudsman Al Hutchinson said there had been no police misconduct and closed his inquiry into the police handling of the affair.
It is understood Dr Maguire ordered his officers to make new inquiries following claims Mr Donaldson was keeping a journal of his time in exile and had connections with a Special Branch officer.
Mr Donaldson, from west Belfast, was living alone in a cottage near Glenties in Co Donegal when gunmen burst in and shot him.
The inquest into his death has yet to take place.
Several people were arrested for questioning, but nobody has been charged with murder.
An Ombudsman spokesman said: "The Police Ombudsman's office can confirm that it is looking at allegations about police conduct in connection with events surrounding the death of Mr Donaldson.
"The office had previously looked at these allegations. Since then, members of Mr Donaldson's family have brought new information to the office which they were not previously aware of.
"The Police Ombudsman, Dr Michael Maguire, has decided that their allegations must be looked at again in light of this new information."
Family solicitor Ciaran Shiels claimed the original ombudsman's investigation failed to interview Mr Donaldson's handler and was unaware of the existence of a journal which he kept, now in the possession of gardaí.
He said: "The family welcomes the new investigation. They would expect that all reasonable lines of inquiry are pursued and they would call on both currently serving and former members of the police to co-operate with the investigation."