A 46-year-old man has been arrested by detectives hunting the killers of Kevin McGuigan.
Police Service of Northern Ireland Detective Chief Inspector John McVea said the suspect was detained in the Antrim area.
He said: “The suspect is assisting officers at the Serious Crime Suite at Antrim police station.”
It is the 13th arrest in the major investigation.
A 41-year-old man arrested on Wednesday remains in police custody.
Mr McGuigan, a 53-year-old father of nine, was gunned down outside his home at Comber Court in the Short Strand area of east Belfast last month.
His death has brought Northern Ireland’s power-sharing political institutions to the brink of collapse.
Mr McGuigan was suspected by some in the republican movement of involvement in the murder of former IRA leader Gerard “Jock” Davison close to Belfast city centre three months ago.
Police believe his killing was a revenge attack by Mr Davison’s republican associates.
Although detectives said individual IRA members were involved in the shooting, there is no evidence to suggest it was sanctioned at a senior level.
PSNI Chief Constable George Hamilton has also said the IRA is not engaged in terrorism but instead is pursuing a peaceful, political republican agenda.
The political fallout from both murders has had major repercussions for the devolved Assembly at Stormont.
On Monday the Ulster Unionist Party withdrew its only minister Danny Kennedy from the five-party Executive amid claims that trust in Sinn Féin had been shattered.
The Democratic Unionist Party (DUP) also held an emergency meeting with the British Prime Minister where they asked David Cameron to suspend the Assembly and convene intensive talks aimed at resolving the crisis.