There has always been hostility between the Provisional IRA and the "Real IRA". While some grassroots members might remain on friendly terms, this is not the case at leadership level.
However, although there have been a series of confrontations between the two organisations since the "Real IRA" was formed, the killing of Joseph O'Connor was the first act of serious violence.
Both dissident and mainstream republicans are bewildered. Mr O'Connor (26), a leading member of the "Real IRA", was shot dead in west Belfast on Friday. No organisation has admitted responsibility but the Provisional IRA is widely believed to be responsible.
Local people said several eyewitnesses saw the gunmen putting on their masks. They were identified as members of a local Provisional unit. It is understood the "Real IRA" knows their names.
"Nothing suggested this was about to happen," a republican source said. "There had been no recent clashes between the two organisations or anything which could in any way have led to this."
Provisional IRA sources said the killing - like any military action during a ceasefire - would have been sanctioned by the army council. "This was not one unit acting independently," said a source.
There appears to be no political "advantage" for the Provisionals. Indeed, the killing is already being used by unionists as another reason to oust Sinn Fein from government.
By shooting Mr O'Connor, the Provisional IRA is risking retaliation from the "Real IRA" on its members and senior Sinn Fein members.
Although, the Provisional IRA remains much more powerful, the "Real IRA" has expanded. Security sources say it is larger than the Provisionals in south Armagh. It has members in Derry and Tyrone and its base in Belfast - particularly Ballymurphy - has grown.
The demand for retaliation is high among "Real IRA" members. However, the leadership is urging calm, viewing the prospect of a feud as "potentially disastrous".
"The only people who would benefit would be the British," said a senior source. Observers believe the "Real IRA" is in a no-win situation. If it doesn't respond to the murder, the Provisionals would clearly have the upper hand and could take further action.
But if it does retaliate, the Provisional IRA would surely respond and could inflict severe losses. At present, grassroots Provisionals are confused. Few would support an offensive against dissidents. That could change if the "Real IRA" retaliated.
"The `Real IRA' will be debating the situation very, very carefully," said a security source. "I don't think there will be any rash actions."
The "Real IRA" was formed in 1997 by senior Provisionals who resigned over the peace process. It launched a bombing campaign but called a tactical ceasefire in 1998 after the Omagh bombing, in which 29 civilians were killed.
However, it has been reorganising and rearming. Security sources say it has secured defections from the Provisional IRA this year because of what is seen as the latter's compromise on decommissioning.
Although the "Real IRA" has not announced the ending of its ceasefire, it has been involved in recent attacks, including the bombing of MI6 headquarters in London. Security sources believe it was preparing to launch a sustained campaign in the North and Britain. A lengthy feud with the Provisional IRA would probably make that impossible.