Senior Sinn Fein member Bobby Storey has been arrested by police investigating a murder by Provisional IRA members.
He was detained today in connection with the fatal shooting of a man in Belfast last month.
Mr Storey is northern chairman of Sinn Fein and one of three men aged 45, 58 and 59, who were arrested in Belfast this morning.
Police have said the murder of Kevin McGuigan was not sanctioned by the IRA.
A police assessment of IRA involvement in the killing has brought the power-sharing institutions in Northern Ireland to the brink of collapse.
The arrests, made in north and west Belfast, take to 16 the number of people detained as part of the murder investigation to date. One man has been charged with weapons offences.
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.
He 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 four 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, they have said there is no evidence to suggest it was sanctioned at a senior level.
Police Service of Northern Ireland (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, with crisis talks on-going at Stormont in a bid to save the stumbling institutions.
Sinn Fein Deputy First Minister Martin McGuinness insisted Mr Storey was a key advocate of peace.
"I was surprised to learn about the arrest this morning of our six-county party chair Bobby Storey," he said.
"Bobby Storey is a valued member of Sinn Fein's core leadership. He has played a leading role in the development of Sinn Fein's peace strategy and is a long standing and loyal supporter, defender and advocate of the peace and political processes.
"We look forward with confidence to his early release."