A letter bomb addressed to Northern Ireland Secretary Theresa Villiers has been discovered at the offices of the First Minister and Deputy First Minister.
The Stormont Castle headquarters of Peter Robinson and Martin McGuinness near Belfast were evacuated, and the army was on the scene.
Two letter bombs were recently sent to the police, one in Londonderry and the other to the chief constable of the Police Service of Northern Ireland. Another was posted to the Public Prosecution Service in Londonderry.
Dissident republicans were blamed.
Mr Robinson and Mr McGuinness recently returned from a trade mission in the US.
Aletter bomb was delivered to the offices of the Prosecution Service in Londonderry on Monday – the third device to be sent through the post.
A stretch of the city’s Limavady Road was sealed off for a time after Army explosives experts were called to the offices at Foyle Chambers, which had to be evacuated by staff.
Last Friday, similar devices were discovered addressed to Police Service of Northern Ireland (PSNI) Chief Constable Matt Baggott and Chief Inspector John Burrows, the police commander in Derry.
Republicans opposed to the peace process have been blamed.
Dissident republicans are also believed to have been behind two other bomb attacks on police last week. Officers escaped injury when pipe bombs were thrown at patrol vehicles in Newtownabbey, Co Antrim, and in Derry city centre.
Dissidents also claimed responsibility for firebombing a shop in Belfast city centre recently.
The threat from extremists opposed to the peace process has been deemed severe by the Home Office. They have been responsible for the deaths of two soldiers, two police officers and a prison guard in recent years.