Officers have arrested a 70-year-old man in connection with the shooting of Denis Donaldson in rural Donegal in April 2006.
The man is understood to be under suspicion of withholding information in relation to the murder, which saw the 56-year-old spy shot a number of times at close range with a shotgun.
The killing occurred just months after Mr Donaldson had been outed as a British spy, and only weeks after gardaí specifically warned him to flee the area as there was a serious threat to his life.
The Sinn Féin insider — who worked in the party’s Stormont offices — emerged as the latest in a line of spies within the Northern Ireland section of Sinn Féin to be uncovered in the years immediately after the Belfast Agreement was signed.
He was a senior back-room member of the organisation.
However, despite the shock of the spy within the ranks, at the time of the find it was suggested in some quarters that other party members may had also been similarly turned.
In 2001, Mr Donaldson was appointed Sinn Féin’s Northern Ireland Assembly group administrator in Parliament Buildings.
In October 2002, he was arrested in a raid as part of a high-profile police investigation into an alleged republican spy-ring — the so-called Stormontgate affair.
In December 2005, the Public Prosecution Service for Northern Ireland dropped the spy-ring charges against Mr Donaldson and two other men on the grounds that it would not be in the “public interest” to proceed with the case.
At a press conference later that month, Sinn Féin president Gerry Adams announced Mr Donaldson was a British spy who had been turned during what was described as a vulnerable point in his life.
In the months that followed he travelled to Donegal, where he was assured his life would not be under threat by republicans. The assurance was made before he was tracked down by a Sunday newspaper journalist.
Within months Mr Donaldson had been killed in an apparent revenge shooting, with the Real IRA claiming responsibility for the murder in April 2009.
His surviving relations have previously said the gardaí murder investigation can only be effective if there is a “thorough and independent examination of the identities, motivations, activities, links, communications and movements of the Special Branch handler” Mr Donaldson knew as ‘Lenny’.
“Lenny is intimately informed about the events surrounding Denis’ murder, Lenny holds the answers to many questions,” they have claimed.