A man previously accused of building the Omagh bomb was last night still being questioned by police investigating dissident republican activity.
Patrick Joseph 'Mooch' Blair was arrested in Newry on Monday.
Blair was once named in the House of Commons as having "helped to construct the Omagh bomb" and taken part in a string of other attacks.
He later denied any involvement with the 1998 atrocity, which killed 29 people.
Police said the 58-year-old is being questioned by detectives investigating dissident republican terrorist activity.
He is being held in Antrim police station.
Blair's arrest came four days after a senior republican, Seamus Daly, was charged with murdering 29 people in the Omagh bombing.
In 2002 Lagan Valley MP Jeffrey Donaldson used Parliamentary privilege to name Blair in connection with the attack.
He said Blair was "a man who is a member of the Provisional IRA and who helped to construct the Omagh bomb".
Blair was also named as a suspect by one-time MI5 informer Kevin Fulton.
In 1975 Blair was sentenced to 15 years in prison for attempted murder. He was released in 1982 and moved to Dundalk.
Blair strongly denies any involvement in the Omagh bomb attack.
In November 2011 he told the Smithwick Tribunal that Fulton was "unreliable" and "a fantasist", and offered to take a lie detector test to prove his case.
"I had no part in any shape or form in the Omagh bombing," Blair told the tribunal. "Kevin Fulton is a liar."