Intelligence takes key role in battle against dissidents
The Police Service of Northern Ireland's decision to stop a van fitted with four primed mortars ready to fire through a hole in the roof was neither a fortunate coincidence nor a spontaneous action.
It will have been what the police describe as "an intelligence-led" operation, meaning they were operating on the basis of information gleaned from an informer or technical surveillance, or a combination of both.
The security service MI5 will almost certainly have played a role.
The police appear to have prevented an imminent attack, with a police station in Londonderry the most likely target.
This is not the first time dissident republicans have deployed mortars in the city.
Just over 12 years ago, in January 2001, the Real IRA launched a mortar attack on Ebrington Barracks, which has since been closed.
One mortar landed inside the perimeter fence of the base after being fired from a parked van.
Other vehicle-borne mortars and some designed to be fired horizontally at passing police vehicles have been recovered by the police, but not on this scale.
"This is not something we haven't seen before," says a security source. "But we haven't seen this kind of multiple mortar device for some years now, so it is a significant and worrying development."
The Provisional IRA used a range of mortars, including multiple launchers and much larger so-called "barrack busters" devices during the troubles.
A launching pad similar to the one recovered in Derry was used by the IRA to attack Downing Street in February 1991 as prime minister John Major presided over a Cabinet meeting.
Two mortars overshot the intended target and did not explode, while the third exploded in the rear garden of Downing Street.
The device intercepted on Sunday night is most likely to have been the work of a new group formed last summer, calling itself the IRA.
It brought together members of the Real IRA, the Derry-based vigilante group Republican Action Against Drugs and a number of non-aligned republicans.
So what does this discovery tell us about dissident republicans?
Security sources have described it as a major "step up" in their technical ability and that will be a matter of concern, both because of the increased threat multiple mortars pose to police stations, and also to any civilians living nearby.
Mortars were notoriously inaccurate and many launched by the Provisional IRA overshot their targets and landed in residential areas.
The discovery clearly demonstrates that dissident groups are developing their technical ability and remain intent on targeting the police.
But it also shows how widely these groups are infiltrated by the police and the security service MI5.
This is the latest in a large number of foiled attacks. Late last year, two more sophisticated tripod launched mortars, similar to those used in Afghanistan, were found in north Belfast and Derry.
Last week police in Belfast recovered a Russian-made rocket launcher, an RPG7, that was almost certainly part of a consignment brought to NI by the IRA from Libya during the 1980s.
There have been other weapons finds and arrests, and many occasions when bombs have failed to detonate.
The murder of prison officer David Black by the group calling itself the IRA in November demonstrated that there is no such thing as 100% intelligence or security and that some dissident activity will succeed.
But, at the moment, the police appear to be disrupting most of what dissident groups are planning.
While that will give the PSNI comfort and cause concern and suspicion in dissident ranks, the police say there there is no evidence that the level of threat they pose has diminished.