A policing oversight body is taking legal action against An Garda Síochána over its failure to comply with an investigation into the murder of British agent Denis Donaldson in Co Donegal almost 20 years ago.
Fiosrú, which replaced the Garda Síochána Ombudsman's Commission (GSOC), has informed the Donaldson family of the move.
In a letter, it said it took the view that gardaí had not complied with a request for information required for its investigation.
Accordingly, it said the ombudsman had decided to commence proceedings in Dublin Circuit Civil Court seeking an order for disclosure, adding that the case was in its early stages.
Both Fiosrú and An Garda Síochána declined to comment.
An Garda Síochána said it did not comment on Fiosrú matters.
The Donaldson's family solicitor, Enda McGarrity, said the Fiosrú investigation had been ongoing for three years.
"We now understand that Fiosrú are commencing legal proceedings against An Garda Síochána arising out of a failure to comply with aspects of the investigation.
"That's a matter of significant concern to the family because the body which is charged with investigating the murder of Denis Donaldson are now not only being investigated, but failing to comply with investigation and that gives rise to family's wider concern that the State are currently not delivering an effective investigation."
Taoiseach Micheál Martin said on Friday that he would be happy to meet the Donaldson family to discuss their concerns.
Denis Donaldson was a senior Sinn Féin official working at Stormont who had been accused of being part of a republican spy-ring there.
He was exposed as a long-standing British agent when the criminal case against him and others was discontinued in December 2005.
He was shot dead in the family's remote cottage near Glenties in April 2006, weeks after he had been photographed and spoken to there by a newspaper journalist.
Three years ago Mr Donaldson's daughter, Jane, complained to GSOC raising concerns about how the gardaí had responded to information about the threat to her father's life.
She claimed that despite being told by the PSNI of an increased risk to Mr Donaldson's life after he was approached by the media, it failed to take adequate measures to protect him.
Dissident republicans claimed the murder, but the family say it has an "open mind" about who was responsible.
Former Sinn Féin leader Gerry Adams successfully sued the BBC after it broadcast a documentary claiming he had sanctioned the shooting.
He had vehemently denied the accusation.
An inquest into the circumstances of Mr Donaldson's murder has been adjourned on 27 occasions due to the ongoing garda murder investigation.
The family is also keen to have their father's journal returned. Gardaí have declined to return it, despite earlier assurances that it would be handed back.
The family believes it may cast light on the lead up to Mr Donaldson's murder.
They would like the Irish Government to establish a commission of investigation which can take evidence in both open and closed hearings.
They believe it is the best way to get to the truth of what happened.
An investigation by Northern Ireland's Police Ombudsman found the PSNI had failed to carry out a formal updated risk assessment on Mr Donaldson after he was visited by the media.
The family is taking a civil action against the police and others on the basis of that report.
