Two men are being held by Irish police over the execution of Sinn Fein official, Denis Donaldson, who was exposed as a British spy.
Donaldson had been working as a British informant for 20 years. He was executed in 2006.
A 30-year-old man has now been arrested along with the 70-year-old man was arrested in Donegal on Tuesday.
Donaldson, an IRA informer and a former close confidant of Gerry Adams was Sinn Fein's senior official in Stormont and a political enforcer for the party leadership. He was shot dead in a cottage in Doochary, County Donegal, in April 2006.
Four months before he was executed he had admitted that he had been a paid informant for the British Government for 20 years. Until now no one was ever arrested in relation to his murder.
On Tuesday the 70-year-old man was arrested under the Offences Against the State Act. They suspected that he was withholding information. Both men are being held in the Letterkenny police station, Donegal.
Last week Donaldson's family asked the police north and south of the border to do more to bring his killers to justice. In a statement issued by their solicitors they claimed that a British security agency had been behind his murder.