The offices of the Northern Ireland First Minister Peter Robinson and deputy First Minister Martin McGuinness were evacuated following a security alert today.
The alert at the Stormont estate in east Belfast began after a suspicious package, thought to have been a viable letter bomb, was found in the internal postal system.
The package was addressed to the Northern Ireland Secretary of State Theresa Villiers.
Ms Villiers is in London today meeting former US diplomat Richard Haass about progress on talks he is chairing with Northern Ireland politicians about boosting the peace process.
She said: "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."
Three other letters bombs have been discovered recently. One was addressed to the PSNI Chief Constable.
Dissident republicans have been blamed.
During the alert, Mr Robinson was moved to Parliament Buildings in Stormont, the seat of the Northern Ireland Assembly.
His work and meetings continued unaffected.
Because the Assembly is in recess,Mr McGuinness was not in Stormont Castle.
Dozens of staff were evacuated during the alert, many of them also moving to Parliament Buildings.
Stormont's Justice Minister, David Ford, thanked staff in Royal Mail sorting offices for their vigilance when handling suspicious packages.
He visited the sorting office at Mallusk, near Belfast , where staff recently intercepted a viable explosive device addressed to the Chief Constable.
On the same day, staff at the Lisburn sorting office intercepted another device addressed to the senior police officer in Derry.
Mr Ford said: "There are delivery offices across Northern Ireland handling many thousands of letters and parcels every day.
Royal Mail has been very active in putting in place security measures to detect and deal with suspicious packages, which in turn helps to secure and protect our citizens.
"I was grateful for the opportunity to personally thank some of the staff involved in dealing with our mail and carrying out those security checks.
There is an obvious threat to these staff when reckless people abuse the mail system and I commend them for their vigilance."