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.
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Mr Cohen is due to appear in federal court in New York at 4pm local time on Tuesday.