Police arrested two suspected Real IRA members today on suspicion of killing Northern Ireland prison guard David Black.
Well-known dissident Colin Duffy, 44, was detained with an unnamed 31-year-old man by police investigating the drive-by shooting on the M1.
No group has claimed responsibility for killing the 52-year-old father-of-two who was on his way to work in Maghaberry prison.
Mr Duffy - from dissident stronghold Lurgan - has successfully defended himself against a series of murder charges, earlier this year he was acquitted of the 2009 killing of two soldiers in Antrim.
He had been held in Maghaberry while remanded in custody. He was part of a group of dissident republican prisoners who have been mounting a dirty protest against strip-searching at the jail.
Presbyterian Minister Tom Greer, a family friend of the Blacks, warned against any sectarian retaliation for the murder from the loyalist community.
And Sinn Fein president Gerry Adams said the actions had been rejected by the entire nationalist community.
"Those organisations that are politically associated with armed groups have failed to outline how these actions advance republican objectives," he said.
"On the contrary, they play into the hands of those in the British system who are opposed to the peace process and to its potential for achieving a united Ireland."
An alleged former IRA leader has been remanded in custody in connection with the 2005 murder of Robert McCartney outside a Belfast bar.
Police said Padraic Wilson was not connected to the murder, but feared he would intimidate witnesses after Mr McCartney's sisters identified him as responsible for leading an internal IRA probe into the fatal stabbing.