Police believe the Continuity IRA (CIRA) was responsible for a bomb found attached to a lorry in County Armagh on Tuesday morning.
It is thought the device may have been intended for a Brexit day attack.
Police said they first received a report about an explosive device in a lorry at Belfast docks on 31 January - the date the UK left the EU.
In a call to a media outlet, it was claimed the lorry was due to travel by ferry to Scotland.
A search was conducted but nothing was found.