The Real IRA has allegedly claimed responsibility for the attack on a British army barracks in Co Antrim in which two soldiers were shot dead and four others injured.
Two pizza delivery men are among those seriously injured in the attack on the Massereene Barracks that left two soldiers dead.
The police officer in charge of investigating the attack has said he has no doubt the attack was an attempt at mass murder.
PSNI investigating officer Detective Superintendent Derek Williamson said at least two gunmen opened fire indiscriminately on a group of soldiers and the two delivery men as they arrived at Massereene Barracks last night.
It emerged that a Sunday Tribune journalist was contacted by someone using a recognised code word, claiming the attack had been carried out by the dissident group.
Journalist Suzanne Breen told Sky News that a male caller claimed responsibility for the shooting on behalf of the South Antrim brigade of the Real IRA.
"He said he made, and the Real IRA made, no apology for targeting British soldiers while they remained what he called occupying the north of Ireland," she said.
Ms Breen said the caller described the pizza staff as "collaborating with British rule" by serving British soldiers. He promised more details of the operation in coming days.
Appealing for witnesses, Chief Superintendent Williamson said: "I have no doubt in my mind this was an attempt at mass murder."
"Last night two very young men lost their lives in a very callous and a very ruthless attack by terrorists who have no thought and had no thought last night for anyone who was in the vicinity.
"Indeed the attack took place at a time when two other young men, two civilians, who were delivering pizzas to the Massereene Barracks stopped, and it is clear from what we know at this stage that the terrorists not only wanted to kill soldiers who were there last night, but also to try to kill those two pizza delivery men."
He added: "The gunmen, having fired an initial volley of shots, moved forward when people were on the ground and fired additional shots at those people on the ground, so it was a very, very callous and very ruthless attack."
The two soldiers who were killed were both aged in their early twenties and were due to fly to Afghanistan on active service in the coming days.
As well as the pizza delivery men, who police also said were very young, two other people were injured in the attack, which has been blamed on dissident republicans.
One of the injured, a Polish national living in Northern Ireland, is critical, two are serious and one is serious, but stable.
Northern Ireland Secretary of State Shaun Woodward said it was clear the shooting was not just an opportunist attack.
"This was a pre-planned attack at mass murder not only on off-duty soldiers but on civilians." He rejected any notion that security around the base had been lax when the gunmen struck.
Mr Woodward said the incident while tragic, did not represent a return to Northern Ireland's dark past and insisted the peace process would go on and would succeed despite the efforts of the dissidents to destabilise it.
"Northern Ireland is now a different place," he said.
"Regrettably, a handful of people remain in the grip of a violent past but we will find them and prevent them doing this in the future."
The chairman and vice chairman of the Policing Board met the Chief Constable this afternoon to discuss the investigation into the attack.
Speaking after the meeting, chairman of the board Professor Sir Desmond Rea described the shooting as "an appalling incident".
"It is now clear that the manner in which this attack took place showed complete contempt for human life. Murder was the only mission," he said.
"This attack has sent shockwaves across our community bringing an unwelcome reminder of our troubled past. The condemnations from right across the community and political spectrum demonstrate unity and resolve that those who carried out this act will not be allowed to undermine the peace process; or the progress that has been made in building a better future for all the people of Northern Ireland in recent times."