GARDAI investigating the murder of IRA informer Denis Donaldson have arrested a second man.
The man, who is in his 30s, was arrested last night and detained under Section 30 Of The Offences Against The State Act at Letterkenny Garda Station, Co Donegal. He can be held for up to 72 hours
A 70-year-old man also arrested yesterday evening remains in custody.
The former senior Sinn Fein officer from west Belfast was shot dead at a remote cottage in Co Donegal on April 4, 2006.
No one has been charged over the 56-year-old's murder.
Just months before his death, Mr Donaldson publicly revealed he had been working as an informer inside the republican movement.
Last week, his family demanded both the gardai and Police Service of Northern Ireland do more to bring his killers to justice.
In a statement, they claimed British security agencies had a part to play in his death.
In October 2002 Mr Donaldson was arrested after police raided Sinn Fein's offices at Stormont, where he worked as the party's head of administration.
The republican spy ring controversy caused the collapse of the devolved power-sharing administration.
But in December 2005 the charges were dropped.
Days later Mr Donaldson was expelled from Sinn Fein after admitting he had been a paid British spy for 20 years.