A letter bomb addressed to the Northern Ireland Secretary of State Theresa Villiers was discovered and made safe by the army at Stormont Castle in east Belfast on Tuesday.
Staff were evacuated after the package was found in the postroom of the building on Tuesday morning. Dissident Republicans are being blamed.
Tuesday’s discovery was the fourth letter bomb to be intercepted in Northern Ireland since last Friday.
Two of the other devices were addressed to police officers and the other was posted to the offices of the Public Prosecution Service in Londonderry.
Deputy First Minister Martin McGuinness, who was not at the estate at the time on Tuesday, said on Twitter: “Letter bombs, attacks on places of Worship, Graves and Orange Halls are the offerings of bitter & twisted little minds & will further nothing.”
On Tuesday, Assistant Chief Constable Drew Harris told the Northern Ireland Affairs Committee that dissidents remained a serious threat, the BBC reports.
"There does not seem to be any diminution in their intent or any sense of them trying to find some route for dialogue with other parties or the government.
"They seem entirely wedded to a route of violence."
He added: "We have concerns about upcoming anniversaries and whether they want to use those for their own purposes in getting publicity."
Chief Constable Matt Baggott said an intense investigation would be launched into finding the letter bombers.
Secretary of State Theresa Villiers said facing terror threats was "sadly still a part of the job".
She added: "I utterly condemn the attempted attacks we've seen over recent days.
"If those responsible think that this kind of criminal activity will further any agenda, then they are completely mistaken."
First Minister Peter Robinson condemned those who sent the letter bomb.
"Those responsible for sending this, and other devices, through the post have absolutely no regard for the lives of postal workers and staff working in offices," he said.
"They will not further any aim or objective by their vile and callous deeds. Northern Ireland will not be dragged back by terrorists who have nothing but misery to offer."