A package in the back of a Royal Mail van driven into Palace Barracks in Holywood contained a fire bomb-style device.
The postal van exploded in flames in a car park of the British army complex - which is also home to MI5 - shortly after 10.30am yesterday, spreading to two cars and three garages.
It is understood the driver had left the van to deliver mail to the reception area of the building when his vehicle burst into fire.
Fire crews battled for about 90 minutes to contain the blaze.
No-one was injured in the attack, but it represents a major security breach at the barracks which has been the target for dissident republicans in the past.
Police forensic experts were yesterday still examining the burnt-out remains of the van to establish the exact nature of the device.
Superintendent Karen Baxter said: "Due to the damage caused by fire and water, the exact nature or make up of the device that caused the explosion has yet to be established.
"Although we have no information to suggest there may be other similar packages in the postal system we are working closely with Royal Mail to mitigate any potential risk."
Royal Mail also confirmed that it was working with the Ministry of Defence and the PSNI to establish how the package was not detected during the sorting process.
One possibility is that the parcel may not have been addressed to the barracks.
Suspicious packages are subject to scanning during the sorting process, with random checks also carried out on mail that passes through.
The mail for the barracks in Hollywood would have been dispatched from the Royal Mail Tomb Street sorting office in Belfast.
The Irish News understands that the postal worker was not approached, hijacked or forced to carry a package against his will at any stage on his morning run.
Police are also checking to see if the van may have been accessed at some stage on Friday morning without the driver's knowledge.
While dissident republicans are thought to be behind the device, no group had last night claimed the attack.
A Royal Mail spokesman said yesterday: “Royal Mail can confirm that there was a fire in one of our vans at the Palace Barracks in Holywood.
"Thankfully our postman was not in the vehicle at the time of the incident and is safe and well.
"We are working with the MoD and Police Service of Northern Ireland on this matter to determine the cause of the fire."
Soldiers from the Royal Scots Borderers have been stationed at the barracks since August last year.
In April 2010 dissident republicans planted a huge car bomb at the entrance to the barracks which caused extensive damage.
A taxi driver was held hostage by three men in Ligonel in north Belfast before being told to drive his vehicle to the base.
Following the attack security was stepped up.
Secretary of State Theresa Villiers described yesterday's attack as futile.
"Those responsible for sending this postal device showed a reckless disregard for the safety of postal workers and others doing their jobs," she said.
"There is nothing to gain from this."
DUP assembly member Alex Easton said he was "deeply angry and concerned that there has been an attempt to kill somebody or soldiers inside Palace Barracks".
"I am just relieved no one was seriously injured or killed and I would appeal to anyone who has any information to bring it to police to put those involved behind bars where they belong."
Ulster Unionist MLA Leslie Cree said the people of Northern Ireland have made "great strides in recent years and they want to see a country where violence and the threat of violence, are left in the past for good".
Alliance Councillor Andrew Muir said he was "appalled and sickened" at the attack.
"I would call on all local politicians to unite in signalling that Northern Ireland has no desire to return to the past," he said.
