A senior south Armagh republican nicknamed 'The Surgeon' has been arrested in an investigation linked to the 2005 murder of Robert McCartney.
Sean Gerard Hughes (51) was arrested by the PSNI's Serious Crime Branch in Jonesborough yesterday morning and taken to Antrim Serious Crime Suite for questioning.
In 2002 DUP leader Peter Robinson used Parliamentary privilege to claim that Mr Hughes had been linked to "many murders on this side of the water and in Northern Ireland".
Mr Hughes is not suspected of Mr McCartney's actual murder, but is the second well-known republican to be arrested as part a police investigation into the IRA's own inquiry into the killing.
Last November Sinn Fein mounted protests at the PSNI's Knock headquarters when Padraic Wilson, a party member, was charged with IRA membership and two counts of addressing a meeting to encourage support for the outlawed grouping. He denies the charges.
Detectives are believed to be focusing on an IRA investigation into the McCartney murder and two meetings the IRA leadership held with his family.
Mr McCartney was stabbed and beaten with an iron bar outside a Belfast bar near St George's Market in January 2005.
A bus-load of Belfast republicans were drinking there after returning from the annual Bloody Sunday commemoration in Londonderry.
Mr McCartney (right) was attacked in the bar and knifed outside it, but many of those in and around the bar said they saw nothing.
None of the republicans present gave evidence and afterwards video footage was removed from the bar, which was also cleaned to remove forensic traces. Sinn Fein also condemned police raids in the Markets area where the killers were thought to have fled.
This led to allegations of a cover-up and, as a result, Mr McCartney's five sisters and his partner Bridgeen Hagans launched a worldwide campaign to bring the killers to book.
The IRA, which had ended its terror campaign, immediately denied that it had authorised the killing. However, it later launched an internal inquiry to find out what had happened.
As part of the inquiry a number of alleged representatives of its 'Army Council' met the sisters on two occasions. The second meeting, in February 2005, lasted five-and-a-half hours, according to a lengthy IRA statement.
In its statement, the IRA gave a full account of the killing, but omitted the names of those involved. It said it knew the identity of four people directly involved in the murder and that two of them were IRA members.
Despite being on ceasefire, the statement said that IRA representatives had offered to "shoot the people directly involved in the killing". It added that the family asked them not to, saying they wanted the culprits to "give a full account of their actions in court".
Two men charged with the murder were acquitted in 2008. The McCartney sisters and Ms Hagans subsequently made statements to the PSNI about the IRA meetings.
Wilson was arrested on October 31 last year and made no comment during questioning. However, the court heard that in a hand-written statement he denied ever being an IRA member or having any involvement in an investigation into the murder.
Last month Robert's sister Catherine McCartney, told the Belfast Telegraph she wanted the IRA to make the findings of its inquiry available to the police.
Last night Ms McCartney said: "I am pleased that the police are continuing to investigate all the circumstances surrounding my brother's murder and I hope that they will reach the truth."
By Liam Clarke
Sean Gerard Hughes was nicknamed 'The Surgeon' by the Army, who blamed him for carrying out strikes against soldiers with surgical precision.
These suspicions were aired at the Smithwick Inquiry in Dublin into the 1989 murders of Harry Breen and Bob Buchannan – the two most senior RUC officers killed by the IRA.
A retired Special Branch inspector said Mr Hughes had organised the killings and had been linked to dozens of other IRA operations.
In December 2002 the DUP's Peter Robinson used Parliamentary privilege to accuse Mr Hughes of having been on the IRA Army Council and also linked him to the 1996 Canary Wharf bombing, which ended an IRA ceasefire and killed two people.
Mr Robinson also claimed that Mr Hughes was responsible for the Warrenpoint bombing in which 18 soldiers were killed, a mortar bomb attack on Newry RUC station that killed nine people, and the killing of Lord Justice Gibson and his wife Lady Cecily. In 2005, The Sunday Times reported Hughes had resigned from the Army Council "after losing interest in the peacetime IRA".
In 2009 Lord Laird used Parliamentary privilege to claim Mr Hughes was one of a number of men who authorised the murder of south Armagh man Paul Quinn in 2007.
However, when the High Court in Belfast froze assets belonging to him in 2009, Sinn Fein's Conor Murphy, who was then the Minister for Regional Development, sprung to his defence.
Mr Murphy said: "I know him very well.
"He's a good friend of mine and has been for very many years, and I'm very proud of that."