Army bomb experts have made safe a viable device sent to the headquarters of Northern Ireland’s police service.
The alert was raised on Thursday after a suspicious package in a brown padded bag with a white address label was received through the post.
PSNI superintendent Sam Donaldson said someone could have been seriously hurt.
“This is a mindless and foolish act that could have resulted in the serious injury of any person handling the package,” he said.
“Although we have no information to suggest there may be other similar packages in the postal system, I would encourage people to be vigilant.”
Last year a group calling itself the IRA said it had sent several letter bombs to Army recruitment offices across the UK, marking the resurgence of a terror tactic adopted by paramilitaries during the Troubles.
Four suspected explosive devices were discovered at Army careers offices in Oxford, Brighton, Canterbury and the Queensmere shopping centre in Slough.
Packets were also sent to military careers offices in Aldershot in Hampshire, Reading in Berkshire and Chatham in Kent.
In 2013 dissident republicans opposed to the peace process were blamed for sending a series of letter bombs to high-profile political and security figures in Northern Ireland.
One of the devices was sent to the seat of the power-sharing Executive at Stormont Castle in Belfast, addressed to Northern Ireland Secretary Theresa Villiers.
Another bomb was delivered to the offices of the Public Prosecution Service in Londonderry, while two explosive packages – one addressed to then PSNI Chief Constable Matt Baggott and the other to one of his senior commanders – were intercepted at Royal Mail offices in Belfast and Lisburn.
The IRA disbanded in the years after it declared an end to its armed campaign in 2005, but a group calling itself the New IRA formed just before the London Olympics in 2012.