The Real IRA last night claimed responsibility for the execution-style killings of two British soldiers outside their barracks in Antrim.
A huge manhunt was underway for the terrorists, who are believed to be operating from Belfast and Lurgan, Co Armagh.
The savage gun attack came just days after PSNI Chief Constable Hugh Orde raised the security threat level in the to "severe".
Senior anti-terrorist officers on both sides of the Border said that initial inquiries and intelligence indicated the culprits were members of the Real IRA, which was responsible for the 1998 Omagh bombing.
Their assessment was backed up by a coded call to a newspaper in which the Real IRA claimed responsibility.
The two victims were collecting a pizza delivery at the Massereene base near Antrim when the gunmen pulled up in a vehicle and opened fire.
Four other people, including two pizza delivery men, were wounded in the shooting.
After an initial burst of gunfire, the attackers shot the victims as they lay on the ground. The two soldiers killed were in their 20s and due to fly out for duty in Afghanistan.
Last night, a 'Sunday Tribune' journalist was contacted by someone using a recognised code word, claiming the attack was carried out by the Real IRA.
Journalist Suzanne Breen said a male caller claimed responsibility for the shooting on behalf of the South Antrim brigade. He told her the Real IRA made "no apology for targeting British soldiers" while they remained "occupying the North of Ireland".
Ms Breen said the caller described the pizza staff as "collaborating with British rule".
Security sources said the involvement of the other main renegade republican organisation, the Continuity IRA, could not be ruled out.
One said: "We have to look at both options, but it seems more likely from what we know that the RIRA group, which has become increasingly active in Belfast and Lurgan in recent months, is behind the attack."
Both organisations are reckoned to have a hard core of about 100 activists in Northern Ireland. Their members are much more scattered on the southern side of the Border because of the inroads made by the gardai.
PSNI Chief Superintendent Derek Williamson said: "I have no doubt in my mind this was an attempt at mass murder."
The two young victims were the first British Army soldiers to be killed in 12 years.
The last soldier to be murdered in the Troubles was Lance-Bombardier Stephen Restorick, a 23-year-old from Peterborough in England.
The two latest victims have not yet been named, but were members of the 38 Engineers Regiment. They were dressed in desert fatigues and were due to fly out of RAF Aldergrove, just a few miles away from their base, in the early hours of yesterday.
One army insider said: "Their plane was due to leave at around 1.20am yesterday morning and this was to have been their last pizza for six months.
"They were fully prepared for war and to face the threat of the Taliban in Afghanistan but these soldiers were gunned down in cold blood before they even had the chance to leave their home base."
The gunmen were aware that soldiers at the barracks followed a regular routine on Saturday nights, when as many as 20 separate orders were made for pizza to be delivered.
One of the pizza delivery men wounded in the shooting was named locally as Anthony Watson (19), who lived in the Antrim area. His condition in hospital is described as serious. It is believed he was shot at least three times.
The condition of the second man, a 32-year-old Pole, is critical. Both worked for Domino's Pizza at Church Street, Antrim, which was open for business last night.
Police are examining a suspect vehicle abandoned in the nearby town of Randalstown at around 11pm last night.
The Massereene attack comes after repeated warnings about the capacity and intentions of dissident groups, by Sir Hugh Orde. It was disclosed last week that the dissident threat level had been officially raised to "severe" -- the second highest alert in Northern Ireland.
This came in the wake of the recent attempted car bombing of a British Army barracks at Ballykinlar in Co Down and an attack on an Orange Hall in Fermanagh. Anti-terrorist officers say there has been some crossover of personnel between the Real IRA and Continuity IRA recently. But they dismissed reports that the dissidents had set up a co-ordinated, cohesive command structure.
Since the Omagh bombing, the Real IRA has split up into several factions. A row in Portlaoise jail between the group's so-called chief of staff, Michael McKevitt, and his second in command, Liam Campbell, resulted in a deep division and the formation of separate outfits.
A third faction, led by a hardliner based in Derry, has been responsible for a series of terrorists attacks, including attempts to murder members of the PSNI, in the past 18 months.