NI prison officers - the threat from dissident republicans
There is a sense of shock at the first killing of a prison officer in Northern Ireland for almost 20 years, but not surprise.
Prison officers have been under threat from dissident republicans for a number of years now. There has been a general threat as well as specific threats against some individuals.
A number have had to move home under a government funded scheme after being informed that dissidents were monitoring their movements and they were being targeted for possible attack.
David Black is not believed to be one of those who had been warned.
"It was just a matter of time," was how one well-placed source responded to the news of the shooting.
The government's assessment of the threat level posed by dissident groups was increased to severe in February 2009, meaning an attack was regarded as highly likely.'Sense of anger'
Since then, dissidents have killed two police officers and two soldiers, and the assumption is that they were responsible for the killing of David Black.
In recent years the police have warned repeatedly that serving officers were the main focus for dissidents. Some prison officers believe the threat to them has been understated.
"There's a sense of anger," said one source. "Prison officers have been worried for some time now that they face an increasing threat, but many feel their concerns have been ignored. Now, they're saying their fears have been realised."
Dissident republicans and their supporters have been at loggerheads with the prison service in recent years in a dispute over the use of strip searching for prisoners entering and leaving Maghaberry high security prison near Lisburn.
There are currently 41 dissidents being held at Roe House in Maghaberry and many are taking part in a "no wash" protest.
Some are smearing excrement on the walls of their cells, others are throwing it out of their cells, and others are refusing to shave.
They say routine strip searching is an unnecessary humiliation and argue that electronic scanners like those used at airports are a secure alternative.
The prison service recently installed two electronic scanners in Magilligan Prison near Limavady and Hyde Bank Wood in south Belfast to test their effectiveness. It has said they may provide an alternative to full body searches.'Growing fears'
It is a long running dispute. Back in August 2010, it was announced that a deal had been reached between the authorities and the prisoners in Roe House to end their protest.
Mediators brokered an agreement that included less restricted movement for prisoners and concessions on strip searching. To facilitate this, a new search facility was introduced and modifications were made to the prison exercise yards.
But the dissidents later accused the prison authorities of breaking the terms of the deal and resumed the protest. Since then, there have been growing fears about possible attacks on prison officers.
A prison service drive to recruit more Catholics may have been another motivating factor for those who killed David Black.
Hundreds of long-serving officers are leaving the prison service under an enhanced redundancy scheme, with their places being taken by new recruits.
Just last week the new head of the prison service, Sue McAllister, expressed disappointment at the low number of applications from Catholics for the new posts advertised and said attracting more would be a priority.
Those who carried out this shooting may hope it deters young Catholics from applying.Dissident groups
In terms of who was responsible, there are a number of possibilities.
The Continuity IRA is active in the Lurgan area and was responsible for murdering Constable Stephen Carroll in March 2009. Just a few weeks ago, two men were arrested on the outskirts of the town in a police operation aimed at the group.
A new organisation calling itself the IRA was formed during the summer, bringing together the Real IRA, Republican Action Against Drugs, and a group of non-aligned republicans - a number of whom are believed to be based in the Craigavon area.
As David Black was from Cookstown, the police will also look at the possibility that dissidents based in County Tyrone may have been involved.