Police in the Republic of Ireland have refused repeated requests to send a murder victim’s secret journal to the Northern Ireland Police Ombudsman.
The journal was being written by the high profile Sinn Féin official and republican informer Denis Donaldson.
It was found by police in the County Donegal cottage where he was shot.
The Ombudsman is investigating allegations that Police Service of Northern Ireland officers may have contributed to his death.
Mr Donaldson’s family believe the journal could contain clues about what happened.
But police in the Republic of Ireland have refused to let investigators have access to the document.
For 20 years, Denis Donaldson led a secret life as an informer for MI5 and the police.
He fled to a remote family cottage in County Donegal in the Republic of Ireland shortly after being told by his police handlers that the media were going to expose his role as an agent.
In April 2006, he was shot dead.
Three years later the Real IRA said it was responsible, but his family have alleged that PSNI officers who knew about his secret role may have exposed him as an agent, and contributed to his death.
They believe the journal, which was removed from the cottage by Irish police officers, could contain potential clues to who killed him and why.
The family was initially told that the journal would be returned to them, but later informed this was not possible for what were described as security reasons.
Ombudman Michael Maguire’s investigators want to read it as part of their inquiry into the allegations that police officers may have contributed to Denis Donaldson’s death.
The Ombudsman has told the BBC that despite the refusal to make the journal available, investigators have uncovered “new and significant lines of inquiry which are well under way.”
A solicitor for the family said they cannot understand why one law enforcement agency will not share information with another.
“The family are of the view it is vital that the police ombudsman receives this journal because they believe it could contain potentially important information,” said solicitor Ciaran Shiels.
“The police ombudsman’s office obviously sees it as a live line of inquiry and it’s extremely disappointing that the garda (Irish police) are obstructing the investigation.
“I think its ridiculous, I think it’s nonsensical, I think it’s completely illogical. The police ombudsman is only attempting to investigate an issue characterised by them as being grave and exceptional and it’s simply beyond the family’s reason as to why the garda would obstruct them.”
An initial investigation by the previous ombudsman, Al Hutchinson, concluded there was no evidence of police misconduct.
But a new investigation was launched last year after the BBC revealed that investigators had not been aware of the existence of the journal, and had not interviewed a Special Branch agent handler the Donaldson family believes could have information about what happened.
The garda have not responded to questions from the BBC about the journal.