The PSNI has described the shooting dead of a former leading dissident republican in west Belfast yesterday as "a brutal and barbaric act."
The attack happened near the Peter Pan centre on Springfield Road in west Belfast.
Giving more details about the murder of Tommy Crossan, the police said the father-of-six was shot dead as he sat at a desk inside a portacabin, at the family fuel business.
Detective Superintendent Jonathan Roberts, said Mr Crossan was shot a number of times in the upper body through the window of the portacabin.
He said Mr Crossan was known to police but said there was no excuse for taking another person's life.
Mr Roberts said a feud among dissident republican groupings was one theory police were examining.
However, he said they were also following a number of other lines of inquiry to try to establish a motive for Mr Crossan's murder.
Earlier, police arrested a 26-year-old man over the murder of Mr Crossan, who was reportedly the former leader of the Continuity IRA.
It is understood that he was recently warned that he was the subject of death threats from his former allies in the ranks of the dissident republicans.
The First Minister Peter Robinson said a small minority would not be allowed to drag Northern Ireland back to the dark days of the past.
The Deputy First Minister, Martin McGuinness, said dialogue, not destruction, was the way forward.
While Belfast's Lord Mayor, Máirtín Ó Muilleoir, said the killers existed in the dark margins of society but could not slow the pace of change