Shankill bomber Sean Kelly was last night being questioned by police investigating the murder of former IRA man Kevin McGuigan.
Kelly was arrested yesterday as police swooped on a number of houses across Belfast.
Republican sources confirmed that Kelly was in police custody but said that they were cynical about the reasons behind the arrest.
The notorious IRA killer murdered nine innocent people when he blew up a fishmongers on the Shankill Road in 1993. He was released on licence in 2000 as part of the 1998 Good Friday Agreement. Five years later he had his licence revoked by the Secretary of State because it was suspected that he had become involved in terrorism once more.
He was released a month later, the day before the IRA ordered a ceasefire in 2005.
Kelly was one of four men arrested yesterday by detectives investigating Mr McGuigan's murder.
The father-of-nine was gunned down outside his home in the Short Strand area of east Belfast.
Two firearms were recovered by police investigating the murder during a number of searches in Belfast yesterday.
A 39-year-old man was also arrested during one of the search operations and taken to Antrim serious crime suite for questioning.
The recovered firearms have been sent for forensic examination as part of the overall investigation. Armed officers swooped on a house in the Lagmore area of west Belfast shortly before midday yesterday as part of the murder probe.
Searches were carried out by police and forensic officers at the property and a car parked in the driveway.
According to eyewitnesses, a man was arrested as he attempted to run from the scene.
The planned searches came shortly after murder squad detectives arrested three men aged 41, 44 and 53 in connection with Mr McGuigan's murder.
Detective Chief Inspector John McVea said the men were arrested at various locations in the greater Belfast area yesterday morning. The were all taken to Antrim serious crime suite for questioning.
Mr McGuigan, a former IRA prisoner, had been questioned, but never charged, over the murder of former IRA leader Gerard 'Jock' Davison in the Markets area of Belfast three months ago.
There has been widespread speculation his killing was a revenge attack by Mr Davison's one-time associates in the IRA. Before his murder, Mr McGuigan had been told by police that he was under threat from republicans.
He was given the warning shortly after the killing of Mr Davison. The two men were once close friends. But they fell out and Mr McGuigan was later shot several times in a so-called punishment attack that was said to have been ordered by Davison.
Stormont's First Minister Peter Robinson has warned Sinn Fein it would face expulsion from the power-sharing Executive if the IRA was responsible for Mr McGuigan's murder. Mr McGuigan's relatives have used social media to accuse the IRA, but Sinn Fein has rejected the suggestion of IRA involvement.