A 48-year-old man arrested by detectives investigating the explosion at Palace Barracks in Holywood has been released without charge.
He was arrested in north Belfast on Friday morning in connection with the 12 April bombing.
The Real IRA bomb exploded outside the Army base which houses MI5's Northern Ireland headquarters.
The explosion happened minutes after policing and justice powers were devolved from Westminster.
An elderly man walking near the barracks at the time was treated for minor injuries.
A number of other arrests have been made over the attack but so far no-one has been charged.