A 42-year-old man has been arrested in connection with an explosion at Palace Barracks in Holywood in 2015.
He has been taken to Musgrave Serious Crime Suite for questioning on Tuesday and released unconditionally on Wednesday.
A device exploded inside a postal van inside the Palace Barracks in August 2015.
The flames spread to two other vehicles and garages within the complex.
The Northern Ireland headquarters of secret service MI5 is based on the site.