Senior republican Bobby Storey last night said he would sue police for wrongful arrest as he was released without charge.
Sinn Fein's northern chairman was released by detectives investigating the murder of Kevin McGuigan after he was questioned for 36 hours at Antrim PSNI station.
Mr Storey's solicitor John Finucane tweeted: "After two days in Antrim, my client Bobby Storey has been freed. No evidence was put at any stage and 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 (59), alleged former head of IRA intelligence, was one of three men who were arrested in Belfast on Wednesday morning as part of the police investigation.
The other two, Eddie Copeland (45) and Brian Gillen (58), well-known republicans, were also released last night.
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.
Earlier yesterday, a woman aged 50 was arrested by police investigating the McGuigan shooting, the 17th person arrested during the inquiry.