Two military personnel have been shot dead during a gun attack at an Army base in County Antrim, police said.
The Police Service of Northern Ireland said two further military personnel and two civilians, all believed to be male, were were in a serious condition.
The incident took place at the Massereene Army base in Antrim, 16 miles north of Belfast, at 2140 GMT.
A spokesperson for the Ministry of Defence described the shooting as a "drive-by" attack.
'Loud bangs'
The four injured men have all been taken to Antrim Area Hospital, about a mile away from the scene.
It is believed that there were two long busts of gunfire during the incident.
A major security operation is under way and the area surrounding the barracks, which is home to 38 Engineering Regiment, has been sealed off.
One witness who lives near the base told the BBC how he looked to the sky after hearing what he thought were fireworks.
He added: "Then I heard a lot of loud bangs again, only it was a lot more than there was initially - maybe between 10 and 20.
"Then the siren at the Army barracks went off. Then all you heard was the police sirens and ambulances and there was at least six ambulances.
“
For the last 10 years, people believed things like this happened in foreign countries... Unfortunately it has returned to our doorstep ”
Ian Paisley Junior
Democratic Unionist Party
"There was definitely six of the ambulances and God knows how many police cars - they just came out of the police station one after the other."
Ian Paisley Jnr, a Democratic Unionist member of the Northern Ireland Assembly and the Policing Board, said the shooting could prove to be a defining moment in Northern Ireland's history.
He said: "For the last 10 years, people believed things like this happened in foreign countries, places like Basra. Unfortunately it has returned to our doorstep."
David Ford, the leader of the Alliance Party, said it was "inconceivable" that the attack could have been carried out by anyone other than dissident republicans.
"The important thing is that we must not see politics and the peace settlement that we have, fragile though it is, destabilised by this kind of action," he added.
No-one has so far claimed responsibility.
The attack comes shortly after Sir Hugh Orde, the chief constable of the Police Service of Northern Ireland, requested the Special Reconnaissance Regiment's help to gather intelligence on dissident republicans.
In 2008, dissident Republicans attempted to kill PSNI officers during separate incidents in Derry City and Dungannon, Co Tyrone.
Security forces defused a 300lb (136kg) bomb in Castlewellan, Co Down, close to a barracks in February 2009.