The family of a murdered police agent have urged the Police Ombudsman for NI to take legal action which would force Irish police (gardaí) to give full access to his journal.
Former Sinn Féin official Denis Donaldson was shot dead in 2006.
This came months after he was exposed as an agent who had worked for the police and MI5 for 20 years.
Allegations by his family that PSNI officers contributed to his death are being investigated by the Ombudsman.
The family claim a journal seized by gardaí from the remote cottage in the hills of Donegal’s north west coast where he was shot has “significant investigative value”.
More than 10 years after the investigation began, the BBC has established that the Ombudsman’s office has still not been given full access to the journal.
Gardaí have told the family the journal will not be returned to them for reasons of “national security”.
It’s understood investigators working for the Police Ombudsman for NI, Dr Michael Maguire, have been given access to some of the contents, but not all.
In a letter to the Ombudsman, a lawyer for the family calls for “direct, urgent and effective action to recover Mr Donaldson’s journal from the possession of gardaí”.
The family also claims the PSNI has refused permission for investigators to speak to a police informer cited as an alibi by an officer they allege contributed to his exposure as an agent and subsequent death.
“These are serious issues for the family which require the Ombudsman’s attention,” solicitor Ciarán Shiels told the BBC.
“The position of the family is that when information that’s clearly vital to the Ombudsman is not being sought, it’s incumbent upon the Ombudsman to robustly tackle the gardaí in relation to the refusal to provide the journal, and also the PSNI if they are halting the flow of sensitive information, and they should be robustly tackling that in the courts of necessary.”
In his letter to Dr Maguire, the family lawyer also claims it has uncovered new evidence of a conspiracy by former and current PSNI officers to cover up their role in his death.
In a response, the Ombudsman has asked the family for details of the information it believes the journal contains.
In a statement to the BBC, the Ombudsman’s office said the investigation was re-opened with the appointment of Dr Maguire in 2012 “and is now drawing to a close”.
It added: “Within the next few weeks we will be approaching the PSNI and An Garda Síochána about a number of outstanding issues.
“Mr Donaldson’s family has indicated it has new information, which we will also need to examine.”
The Donaldson family has requested an urgent meeting with Mr Maguire to discuss their concerns.