Secretary of State Theresa Villiers is under pressure to revoke Shankill bomber Sean Kelly's early release licence after his arrest over a man's murder.
Kelly was detained by police for questioning over the killing of 31-year-old Conor McKee who was shot dead in his north Belfast home in January. He was released without charge last night.
Police said the father-of-two Mr McKee was the victim of a "particularly brutal murder".
Kelly, whose bomb killed nine people in 1993, plus fellow bomber Thomas Begley, was released on licence under the Good Friday Agreement.
In light of his arrest yesterday - the second murder he has been arrested in connection with in the space of eight months - unionists have demanded that his licence be revoked.
In August, the 42-year-old north Belfast man was arrested in connection with the murder of Kevin McGuigan Snr, a 53-year-old father-of-nine, who was gunned down in east Belfast in a suspected feud between former IRA members.
Kelly was later released without charge.
At the time, he claimed that he was just being used as a "political pawn".
DUP Policing Board member Jonathan Craig said that Kelly should never have been released from prison in the first place and called for him to be returned behind bars.
"Sean Kelly was a mass murderer," he said. "His early release, along with the others at that time, was very difficult for people to stomach. If there is the slightest hint of suspicion that he has been involved in murder then his licence should be revoked."
Ukip's David McNarry added: "As far as I am concerned, his licence should be revoked immediately. The Secretary of State now has to show Northern Ireland that security comes before politics."
Independent Belfast councillor Ruth Patterson called for "clarity" about the terms of Kelly's licence.
"The PSNI clearly have intelligence that suggests that this mass murderer continues to be involved in terrorism and crime," she said. "We need some answers around the terms of Sean Kelly's licence."
Kelly was given nine life sentences for the Shankill bombing, but was released in July 2000 under the early release terms of the Good Friday Agreement.
In July 2005, his licence was revoked by the then Secretary of State for Northern Ireland, Peter Hain. There was evidence, said Mr Hain, that Kelly had become "re-involved in terrorism".
Ten days later, Kelly was released on the orders of Mr Hain in a move strongly condemned by unionists. He was freed the day before the IRA issued a statement ending its armed campaign. In 2013, Kelly was arrested by detectives investigating the shooting of an 18-year-old man in the Ardoyne area of north Belfast. He was later released unconditionally.