Sinn Fein's northern chairman Bobby Storey has been released by detectives investigating a killing by suspected members of the IRA, the Police Service of Northern Ireland (PSNI) said.
He was one of three leading republicans questioned about the shooting dead of Kevin McGuigan in Belfast last month.
The disclosure that Provisionals are believed to be behind the death has rocked the political institutions, with Democratic Unionist First Minister Peter Robinson resigning today.
Mr Storey's solicitor John Finucane tweeted: "After 2 days in Antrim my client Bobby Storey has been freed. No evidence was put at any stage & my client will be suing for unlawful arrest."
Senior officers have said the murder of Mr McGuigan was not sanctioned by the IRA, but one major line of inquiry is that members of the PIRA were involved.
It is 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, alleged former head of IRA intelligence, was one of three men, aged 45, 58 and 59, who were arrested in Belfast on Wednesday morning as part of the police investigation. He is a former IRA prisoner.
The other two, Eddie Copeland and Brian Gillen, are also well-known republicans.
Sinn Fein president Gerry Adams welcomed Mr Storey's release.
"The unconditional release of Bobby Storey underlines the contrived nature of the current crisis in the political institutions in the North," he said.
"I want to reiterate my grave concern about the nature in which the murder of two men has been exploited and also at the way the current difficulties have developed in the last few weeks, including the arrest of Bobby Storey."