Northern Irish police said Thursday they had found a bomb attached to a truck which was intended to go off as the UK left the European Union.
The Continuity IRA (Irish Republican Army), a dissident paramilitary group, were blamed for planting what detectives said was a viable device.
Dissident republicans like the Continuity IRA seek Northern Ireland's integration into the Republic of Ireland through violent means.
Police said a sketchy warning call was made to a media outlet on January 31 -- the day the United Kingdom left the EU -- about a device on a truck in Belfast docks, due to take a ferry to mainland Britain.
Searches were conducted and nothing was found.
But on Monday, a more detailed warning call said the device had been attached to a truck trailer belonging to a particular haulage company.
It was found inland at an industrial estate in Lurgan, southwest of Belfast, and was made safe by British army bomb disposal experts.
Police believe the Continuity IRA thought the trailer was destined for a ferry to Scotland but had selected the wrong vehicle as the one containing the bomb did not leave the premises at all.
The bomb "could have caused death and very serious injury," said the Northern Irish police's temporary assistant chief constable George Clarke.
Those who created the bomb "did intend that the device would explode at around the time the UK left the EU", he added.
The UK deems the Northern Ireland-related terror threat level to be severe -- the second-highest of five levels -- meaning an attack is considered "highly likely".
A solution had to be found that would keep the UK's only land border free-flowing between Northern Ireland and the Republic of Ireland, an EU member state.
An open border was a provision of the 1998 Good Friday Agreement that largely ended more than three decades of violence in Northern Ireland.
In a transition period, Britain will adhere to EU rules until the end of the year, when a controversial dual customs system is introduced in Northern Ireland.
The Republic of Ireland has a general election on Saturday. Mainstream republicans Sinn Fein topped the last opinion poll.
Northern Ireland's police chief Simon Byrne gave the force's oversight body a briefing on the incident at their monthly meeting on Thursday.
"This could have ended up on a ferry," Sinn Fein board member Gerry Kelly told the UK's Press Association news agency.
"If it had exploded, you are talking about catastrophic loss of life.