The wedding next week of Prince William and Kate Middleton presents a tempting target for a small group of resurgent Irish terrorists responsible for more than 40 significant terror attacks over the last year, senior British security officials tell ABC News.
While there is no specific threat of any organized plot against the wedding, officials say there is "concrete intelligence" that one Irish group, calling itself the Real IRA, has been trying to move its operations beyond Northern Ireland to London.
"It is fair to assume that people are worried about an attack on the mainland," said Mark Hamilton, chief superintendent of the policed service in Northern Ireland.
The British officials in charge of countering domestic terrorism, MI5, have recently increased surveillance of suspected Irish terror leaders, shifted resources from al Qaeda to the Irish groups and raised the threat level of Irish-related terrorism from moderate to substantial.
"There is a massive effort by the intelligence services and the police to track these people down," said superintendent Hamilton.
Largely unnoticed outside of the United Kingdom, splinter groups of Irish Republican militants have increasingly ignored the peace agreement reached 13 years ago this week that was meant to end IRA violence.
"I think these people would have no compunction at all about carry out an attack, if they could, on an event like the royal wedding," said Professor Martyn Frampton, a history lecturer at Queen Mary, University of London, and the author of a new report, "The Return of the Militants: Violent Dissident Republicanism."
"Attacks on the mainland, particularly here in London, I think, remain the holy grail for these organizations," said Frampton in an interview broadcast Monday on Good Morning America.
"They would say it is an opportunity to target the British establishment," Frampton added.
Security officials say there are several splinter Irish groups that continue to target police officers and other targets that symbolize their perceived enemy: British rule of Northern Ireland.
The Arming Switch. Irish terror cells traditionally use an assembly-line method to manufacture bombs, timers, and mortars, so authorities fear there are at least a couple of other working devices out there ready for use. Exclusive pictures of potential devices were obtained by ABC News.
The Real IRA
Security officials in England and Ireland tell ABC News they believe there are about 600 members of the real IRA and related splinter groups including the Continuity Irish Revolutionary Army.
The groups have demonstrated an ability to gather powerful weapons and have displayed a surprising skill in making new kinds of bombs, officials told ABC News.
Most troubling, officials say, is the use of rocket or mortar launchers capable of sending an explosive device of 40 pounds over security walls more than 300 yards in the distance.
Police have seized large quantities of high explosive and weapons in recent raids, according to police superintendent Hamilton.
"When people have that type of weaponry in communities, then the intention clearly is to cause devastation to somebody," he said.
Most recently, a 25-year old Catholic police officer in Northern Ireland, Ronan Kerr, was killed by a booby-trap bomb hidden in his car in the village of Omagh, outside Belfast.
That device, ABC News has learned, appears to have been composed of just a few ounces of powerful plastic explosive placed in an aerosol can affixed under the officer's car.
The Irish terrorists have a long record of attempting attacks on the royal family.
Prince William's parents, Prince Charles and the late Lady Diana, were targeted by an IRA bomb plot in 1983 as they attended a London concert of Duran Duran.
In 1979, an IRA bomb killed Lord Mountbatten, the Queen's cousin and a mentor to Prince Charles.
The resurgent groups, while smaller in number and enjoying substantially less popular support, have similar ambitions, said Professor Paul Bew, a professor of politics at Queen's University in Belfast and a recognized expert on Irish dissident violence.
"It must be the ambition to repeat some of the dramatic explosions that happened, for example, in London during the IRA campaign," Bew told ABC News.
"They are a much less potent force, but they can do it," he said. "And therefore everybody has to take on board the fact that there is something new in play here."
And Bew says because the groups, in fact, have less popular support, they may feel more emboldened in their targeting.
"They would not be constrained by a view which said, 'That's going too far,'" said Bew. "They're more of a wild card, they're more unpredictable."
The mortar tube is plastic, and the bombmaker has used the gas cylinder from a car airbag as the propellant. Earlier models required the mortar to be within 30 to 40 yards of the barracks or police station. Image obtained exclusively by ABC News.