The suspect packages sent to Army Careers offices were in A4-sized envelopes, postmarked Ireland, contained black powder which was explosive and if ignited would have caused flash burns, ITV News' Juliet Bremner understands.
However, the officers who discovered the suspect packages are not certain that they would have exploded.
This indicates that Irish dissident groups were very likely behind the attack.
More top news
The friends of a Czech businessman who was beaten to death raise £10,000 in a day to send body of 'great man with a big heart' back home
The Olympians rounded off their success in Rio with a personal triumph as they confirmed they have tied the knot.
The foreign secretary called for tougher action against Russia in an effort to halt the "barbaric" attacks on rebel-held Aleppo.