Detectives investigating the murder of Denis Donaldson, a British spy inside the IRA, have arrested a man.
The suspect, aged in his 40s, is being questioned at Letterkenny Garda Station, Co Donegal, about the April 2006 killing.
He is being held under section 30 of the Offences Against the State Act.
Mr Donaldson, 55, a Sinn Fein official and close colleague of party president Gerry Adams, was shot dead at an isolated cottage near Glenties in Co Donegal.
Last week, an elderly man from Scotland was granted bail after he was charged with withholding information in relation to the same investigation.
Another man, in his 40s, was released without charge while a file was being prepared for the Director of Public Prosecutions.
Mr Donaldson had been living in a remote area of Donegal following his exposure as an MI5 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 assassination have long been shrouded in mystery.
A long-delayed inquest into the shooting has been adjourned almost 20 times.
Gardai have repeatedly urged the coroner to postpone the probe, citing concerns it might compromise their criminal investigation.
In 2014, the Garda made a mutual assistance request to a police force outside the Irish Republic in a bid to gain what it described as potentially "significant" evidential material.
That material was secured in March this year.
Investigating gardai have reissued an appeal for anyone with information about the murder to get in contact.