Police received their first reports of shooting at Massereene barracks near Antrim at about 2140 GMT on Saturday night.
Earlier in the evening, soldiers at the base had ordered pizza from Domino's Pizza  in nearby Antrim.
Two men left the shop in cars at about 2120 GMT to make the delivery.
When they arrived at the main gate of the barracks, just off Randalstown Road , four soldiers came out to collect the food.
The attack took place outside the main gate of the barracks
As they did so, two gunmen in a car parked nearby opened fire with semi-automatic rifles.
Sappers Mark Quinsey, 23 and Cengiz 'Patrick' Azimkar, 21, were killed and another four people seriously injured, including the pizza delivery men.
Police say they were an innocent party in the attack.
After the initial volley of gunfire the attackers then approached their victims, who were lying on the ground, and opened fire again.
A Vauxhall Cavalier which police believe was used by the gunmen, was later recovered in Ranaghan Lane near Randalstown .
During the Northern Ireland conflict, the republican paramilitaries warned they would kill anyone who worked or provided services to the police and Army.
Victims included petrol station staff, cooks and caterers and construction workers.
Massereene barracks is the headquarters for the Northern Ireland regiment of the army's Corps of Royal Engineers.
The regiment gives technical support to other army units, including communications and construction expertise. Many of its soldiers are currently deployed in Afghanistan.