A man in his 40s has been arrested in connection with the murder of IRA informer Denis Donaldson.
Mr Donaldson, a former leading Sinn Féin member, was shot dead in the Doochary area of Donegal in April 2006.
It happened months after he was exposed as an agent who had worked for MI5 for over 20 years.
The person arrested today is being detained at Letterkenny garda station under section 30 of the Offences Against the State Act 1939, as amended.
It comes a week after a man in his 70s was charged with withholding information from the garda investigation into the killing.
Patrick Gillespie, with an address at Craigvar Street in Glasgow, was granted bail by the Special Criminal Court last Friday.
Anyone with information has been asked to contact gardaí in Letterkenny at 074 9167100, the garda confidential line at 1800 666111 or any garda station.
