Gardaí are continuing to question a man in his 40s in connection with the murder of Denis Donaldson in Co Donegal five years ago.
Mr Donaldson, a former Sinn Féin official who had admitted being a British agent for more than 20 years, was shot dead just outside the town of Glenties in 2006.
The man is being detained at Monaghan Garda Station.
It is the third arrest in relation to the ongoing investigation.
