A 42-year-old man arrested by detectives from Serious Crime Branch investigating an explosion at Palace Barracks in Holywood in August 2015 has been released unconditionally.
He had been taken to Musgrave Serious Crime Suite for questioning yesterday.
A small device exploded in the back of a postal van inside Palace Barracks on August 14, 2015.
No-one was injured in an attack blamed on dissident republicans.
The Northern Ireland headquarters of MI5 is based on the site.