By Ian Graham
BELFAST (Reuters) - Northern Ireland police foiled an attempt to fire mortar bombs at a police station overnight in what would have been the first attack of its kind in the United Kingdom since a peace deal ended the Irish Republican Army's campaign of violence.
Officers said they were working on the assumption smaller Irish nationalist militant groups were behind the planned assault, though no group claimed responsibility.
Police said they intercepted a white van on the outskirts of Londonderry on Sunday at 3.15 p.m. ET carrying four mortar bombs that were minutes from being deployed. After 100 homes were evacuated, army sappers disarmed the bombs.
"We could have been looking at mass murder today if those devices had exploded and hit their intended target," Chief Superintendent Stephen Cargan told journalists.
"It was certainly sophisticated and worrying in terms of the capability," he said.
A 1998 peace deal largely ended more than three decades of violence in the British-controlled province between mainly Catholic Irish nationalists seeking union with Ireland and predominantly Protestant unionists who want to remain part of the United Kingdom.
But dissident nationalists, who include former operatives who split from the Irish Republican Army after it declared a ceasefire, still stage sporadic gun and bomb attacks.
The threat has intensified in the past four years as frustration with the power-sharing government established under the 1998 peace deal has grown on the fringes of the nationalist community.
ECHOES OF IRA
Irish nationalist militants have used mortars before, most spectacularly when the Irish Republican Army in 1991 fired a shell into the garden on Downing Street, exploding within 50 feet of then British Prime Minister John Major and his cabinet.
The thwarted attack would have been the first by dissident nationalist using multiple mortars, a police spokesman said. While police described the mortars as "crude home-made devices," they said they could have caused extensive damage.
Police in Belfast last week seized an RPG 7 rocket launcher and warhead, a weapon more associated with the IRA than the dissidents, who have largely relied on home-made explosives.
"This sends out message they are increasing their capacity, which is a worrying development," Doctor Jonny Byrne, a lecturer in criminology and politics at the University of Ulster.
"It raises the question of how much of this is coming from former IRA operatives who may be passing that on to a new generation," he said.
Police said they believed the van, which had a hole in its roof to allow the mortars to be fired from inside, had crossed the border from neighboring county Donegal in north-west Ireland.
It was stopped close to the main police station in Londonderry, Northern Ireland's second city. Police arrested three suspects, one in the van, one on an accompanying motorbike and a third in a separate a raid soon after.
The attack was condemned by mainstream Irish nationalist politicians, including Sinn Fein which was once the political wing of the now defunct Irish Republican Army.
While support for the militants is relatively weak in the nationalist community, Monday's attack will renew concerns about the security at a meeting of world leaders at a G8 summit in Northern Ireland in June.
"We continue ...to be extremely vigilant in the face of what is a very, very severe threat," Northern Ireland's Secretary of State Theresa Villiers said in an interview with Sky News.
Militants have targeted Londonderry in recent months in the run up to the city's 2013 term as UK City of Culture.
There is also discontent with Northern Ireland's power-sharing government in working-class Protestant areas, which fuelled weeks of rioting in December and January after nationalist councilors voted to end a century-old tradition of flying the British union flag every day over Belfast City Hall.
(This version of the story corrects the day and time of police interception.)
(Writing by Conor Humphries and Stephen Mangan; Editing by Andrew Heavens)