Dissident republicans planted a bomb on a lorry they believed was bound for an Irish Sea ferry with a plan to detonate it to mark Brexit, the PSNI have said.
Officers have blamed the Continuity IRA for the botched terror bid last Friday night.
Police Service of Northern Ireland Assistant Chief Constable George Clarke said the “carnage” that could have been caused if the device had exploded was worrying to contemplate.
The renegade group entered the yard of a Co. Armagh-based company specialising in the transportation of frozen goods and attached the bomb to a heavy goods vehicle they thought was destined for a late-night ferry crossing to Scotland.
But officers suspect the dissidents selected the wrong vehicle, as the trailer containing the bomb did not leave its premises in Lurgan on Friday.
The device was finally discovered at the yard on Monday night after an intensive police search operation.
The security alert was initially prompted when the Belfast newspaper the Irish News received a warning on Friday night that a device had been left on a trailer in Belfast docks.
Mr Clarke said the warning was “sparse and limited”.
He said it claimed that the bomb would be on the midnight ferry, when there was no ferry scheduled to depart at that time.
A major police search operation instead focused on a late-night Stena Line ferry to Cairnryan. When nothing was found, the ferry was allowed to sail at 11.16pm.
The Irish News then received a further call on Monday.
“That call contained substantially more detail,” said Mr Clarke, who is the PSNI’s lead officer on Brexit.
Mr Clarke declined to be drawn when asked at a media conference in Belfast whether the device would have had the capacity to sink the ferry.
“I am not going to get into a discussion about the engineering of the device but I will make a very simply point – anybody who plants a device in a public place is reckless to the consequences of their actions and of the potential to kill or seriously injure people in that area,” he said.
“This is an incredibly reckless activity.”
Police have now stepped up their presence, both uniformed and undercover, at ports in Northern Ireland amid fears of another attack.
Stena Line declined to comment due to the live police investigation.