Kevin McGuigan murder: Bobby Storey and three others held
Senior republican Bobby Storey is one of four people now being questioned over the murder of former IRA man Kevin McGuigan Sr.
Mr McGuigan Sr was shot dead at his home in east Belfast last month, and police said members of the Provisional IRA were involved in the killing.
The men, aged 59, 45 and 58, were arrested in north and west Belfast on Wednesday.
A 50-year-old woman was also arrested in Belfast on Thursday.
The other two men arrested on Wednesday are Eddie Copeland and Brian Gillen, both leading republicans.
All four are being held at Antrim police station.
Police will have to go to court on Thursday evening to apply for an extension if they want to hold the three men for more than 48 hours.
Mr Storey, a former IRA prisoner, is the northern chairman of Sinn Féin.
In 2005, Ulster Unionist Party (UUP) MP David Burnside told the House of Commons under parliamentary privilege that Mr Storey was the head of intelligence for the IRA.
Mr Storey is a close ally of Sinn Féin president Gerry Adams and has an office at Stormont.
The Police Service of Northern Ireland's Chief Constable George Hamilton said last month that Provisional IRA members had a role in Mr McGuigan Sr's murder.
But he added there was no evidence at that stage that the killing had been sanctioned at a senior level of the organisation.
Sinn Féin said the IRA no longer exists after it ordered an end to its armed campaign in 2005.
A total of 17 people have been arrested in connection with Mr McGuigan Sr's murder.
One person has been charged, while the others were released unconditionally.