It has emerged that the man arrested by gardai in connection with the murder of Denis Donaldson is leading Derry dissident Gary Donnelly.
Donnely, who is in his 40s,was arrested in County Monaghan on Saturday.
Denis Donaldson was shot dead at his remote cottage near to Glenties in April 2006.
Just months earlier the former Sinn Féin official publicly admitted being a British agent inside the republican movement for more than 20 years.
Yesterday’s arrest is the third to be made as part of the ongoing investigation.
In April two men , they were subsequently released with out charge.