By Vincent Kearney
BBC NI Home Affairs Correspondent
The bomb left at Palace Barracks was placed in a taxi which was hijacked in north Belfast
The threat from dissident republicans in Northern Ireland is higher than at any time since the Omagh bomb almost 12 years ago, according to police.
Senior police officers believe rival factions in the Real IRA and Continuity IRA have increased co-operation and stepped up recruitment.
There is concern that they may try to disrupt the election campaign.
Security sources have told the BBC the dissident situation has "deteriorated significantly".
The threat posed by dissidents has been officially classified as "severe" for the past 14 months.
The assessment was increased from "substantial" last February.
This means the security service MI5, which is responsible for gathering intelligence believes an attack is highly likely.
While that official threat level has remained unchanged since then, security sources have said the situation has got more dangerous.
"It's a pretty bleak assessment," one senior source said,
"It's not looking good at the moment and isn't likely to improve in the foreseeable future."
Not only have the numbers involved in dissident groups increased, so too has the level of technical skills and co-operation.
The Real and Continuity IRA have always been described by security sources as separate groups working independently of each other, and even with rival factions inside organisations. But that assessment has now changed.
Senior police sources said there is evidence of increasing communication and co-operation between dissident factions, working together on logistics, weapons, planning and carrying out attacks, and that has increased the threat they pose.
Dissidents have carried out 10 attacks this year, but the police have prevented many more potential attacks.
Intelligence suggests that about 50 dissident "operations", including targeting and planning, have been disrupted this year.
The increasing co-operation was evident in the car bomb attack on Palace Barracks, in Holywood, the headquarters of MI5, shortly after midnight on the 12th of April, just minutes after policing and justice powers were devolved to the Stormont Assembly.
The attack also demonstrated improved technical ability that is causing concern in security circles.
Dissidents have tried to mount a number of bomb attacks in the past, but failed for a variety of reasons.
A car bomb abandoned outside Castlewellan in January 2009 was defused; a landmine in Forkhill didn't explode; neither did a number of devices in County Fermanagh; and then a car bomb placed outside the headquarters of the policing board in November 2009 only partially exploded.
Then a 250 lb car bomb exploded outside Newry courthouse in February followed last week by the bomb at the gates of the offices of MI5.
Police believe those two devices, and the car bomb at the policing board, have a common thread - they were all constructed by bomb makers in the border area and then passed on to other dissidents who attached the final components, like firing packs and timer units.
Security sources believe dissident bomb makers in the border area have now perfected their techniques after a period of research and development, and are supplying other dissidents with "ready to use packages".
Police sources describe the south Armagh and north Louth area as the "crucible" of dissident republican bomb making.
The car bomb used in Newry simply had to be driven a short distance across the border, but the car placed outside the policing board, packed with an oil drum that contained around 400 lbs of explosives, was driven to Belfast and then handed over to those who later planted it.
The device that exploded outside Holywood barracks, which contained around 100-120 lbs of explosives, was brought to Belfast in a beer barrel and then placed inside a hijacked taxi.
There is not a centralised "engineering department" like the Provisional IRA operated during the troubles, and the police don't know how many bomb makers are involved.
The security services believe dissidents pose a greater threat
What they do know is that technical problems that may have prevented previous bombs exploding appear to have been overcome.
Police believe some of those involved are former members of the Provisional IRA as the car bombs are similar to devices used in the early 1990s.
Significantly, police sources said none of the car bombs have contained Semtex or any other commercial explosives as they are built in a way that doesn't require a booster charge.
"This is old school," one source said.
"There is no evidence of new developments or improved technology.
"We believe they've been researching, developing and testing previously used techniques and have now got it right."
The threat is now regarded as the most severe since a Real IRA bombing campaign in 1997-1998, when large devices exploded in a number of towns, including Banbridge and Moira, before 29 people were killed in the attack in Omagh in August 1998.
The threat against individual police officers also remains high.
The vast majority of the 50 or so dissident "operations" disrupted this year involved plans for attacks on officers, and dissidents are said to be increasingly targeting officers on duty in a bid to make it impossible for them to operate on the streets in some areas.
Getting more officers on to the streets and into communities is a stated policy objective of Chief Constable Matt Baggott.
Police activity has increased in response to the growing threat. Already this year 64 people suspected of involvement in dissident activity have been arrested, and 18 have been charged - compared to 17 charges for the whole of last year.
Detectives hope to learn lessons from the past.
They are looking back, in some cases almost 20 years, to see if they can find forensic links between the recent bombings and previous attacks, and identify the bomb-makers.
Police are also investigating the finances and other activities of individuals they believe are involved in dissident activity.
The threat is likely to increase further in the weeks ahead as dissidents are expected to step up efforts to mount attacks during the election campaign.