A core member of Sinn Fein's leadership and alleged former head of IRA intelligence has been arrested by police investigating a murder linked to Provos.
Bobby Storey is northern chairman of the party and was one of three well-known republicans detained today in connection with the fatal shooting of Kevin McGuigan in Belfast last month which has brought the power-sharing institutions in Northern Ireland to the brink of collapse.
Sinn Fein Deputy First Minister Martin McGuinness said 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.
"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 longstanding and loyal supporter, defender and advocate of the peace and political processes.
"We look forward with confidence to his early release."
Police have said the murder of Mr McGuigan was not sanctioned by the IRA.
But they said one of the major lines of inquiry is that members of the Provisional IRA were involved in the murder.
It has been alleged to have been carried out in retaliation for the murder of another former IRA prisoner, Gerard "Jock" Davison, at his home in the Markets area of Belfast in May.
Mr Storey was one of three men, aged 45, 58 and 59, who were arrested in Belfast this morning as part of the police investigation.
In 2005 former Ulster Unionist South Antrim MP David Burnside told the Commons: "The Secretary of State knows the facts from his security information. He knows about Bobby Storey, head of intelligence, reporting to the army council."
Sinn Fein president Gerry Adams also reacted to the arrest, describing Mr Storey as a valued member of the party leadership, a person of great integrity and someone he trusts.
"He's been charged with nothing. I have grave concerns about how all of this current crisis has developed since the dreadful killing of Kevin McGuigan, and am mindful in recent times that two families, the Davison and McGuigan families, have both been grieved," Mr Adams said.
The Sinn Fein leader claimed the potential for a breakdown of the talks at Stormont House was more to do with ongoing competition for support between the Ulster Unionists and Democratic Unionists.
Mr Adams added: "At this stage I don't want to go any further than saying I have grave concerns about how all this has developed, including the arrest of Bobby Storey."
Sinn Fein said it was their understanding that DUP leader Peter Robinson had given Northern Ireland Secretary Theresa Villiers "an ultimatum" to suspend the Stormont institutions or the party's ministers would be withdrawn following the Storey arrest.
Subsequently the DUP sought an adjournment of Assembly business.
The McGuigan 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 Davison murder.
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 ongoing at Stormont in a bid to save the stumbling institutions.