Dissident republicans behind a spate of letter-bombs and security alerts will never succeed in their political aims, the Secretary of State has vowed.
Four explosive packages have been intercepted in less than a week – with nervousness over the spate causing two extra alerts yesterday.
One viable device was addressed to Theresa Villiers (below) and was discovered by staff in the post sorting room at Stormont Castle on Tuesday.
It followed similar attempts on the lives of PSNI Chief Constable Matt Baggott and one of his senior commanders. An explosive device was also sent to the Public Prosecution Offices in Londonderry.
The alerts come on the back of a series of bomb alerts – some hoaxes and some real – which have caused disruption in many locations in Northern Ireland.
During a series of public engagements yesterday, Ms Villiers described the recent upsurge in dissident republican activity as "disgraceful".
Ms Villiers met with Royal Mail workers and postal staff at Stormont Castle to hear about their experiences.
"I wanted to say thank you to them and also provide reassurance that all focus is on trying to track down the people responsible for these attempts, to frustrate them and prevent them from carrying out their intentions to harm people in Northern Ireland," she said.
The Conservative MP said it was a huge source of frustration the spate of letter-bombs had prompted disruption to everyday life.
She added: "It is also economically damaging and these attacks will never succeed in their political aims.
"The vast majority of people in Northern Ireland have chosen the democratic route, they won't be forced onto another pathway by violence or hoaxes or by the sort of disgraceful attempted attacks we have seen in the last few days."
In recent weeks there have been dozens of security alerts including pipe-bomb attacks on police officers in Belfast and Londonderry.
The heightened sense of apprehension led to two security alerts yesterday.
An alert at the law courts in Belfast ended after a reported suspicious package was found to be a legitimate delivery. Staff had contacted police amid fears the item was another letter-bomb.
Hours later the postal clearance station on the Stormont Estate was evacuated while a package addressed to a member of staff was examined. It was found to be harmless.
On Tuesday, Stormont Castle was evacuated after a viable letter-bomb addressed to Ms Villiers was found. She was not on the site when the letter-bomb was found, as she was in London to meet US diplomat Richard Haass, who is chairing inter-party talks.
Last week, two explosive packages, one addressed to Chief Constable Matt Baggott and the other to Chief Inspector Jon Burrows, were intercepted at Royal Mail offices in Belfast and Lisburn.
Justice Minister David Ford praised postal staff for their vigilance.
Condemning the threats, First Minister Peter Robinson said those responsible "have absolutely no regard for the lives of postal workers and staff working in offices". Those behind the packages were lambasted as "bitter and twisted" by Deputy First Minister Martin McGuinness.
Royal Mail, which handles some of the post at Stormont and discovered the letter-bombs in its sorting offices last week, said it had "strict procedures" in place to deal with suspect packages. "Our employees are regularly briefed on these protocols which are under constant scrutiny and review. The safety of our people is our number one priority," a Royal Mail spokeswoman said.
Last week, Mr Ford told the Assembly a terrorist attack on the security forces or the public in the run-up to Christmas is "highly likely".
Mr Ford told MLAs the terrorist threat level remains severe "which means that an attack is highly likely. There have been 14 attacks up to October 14 this year (with) police officers, soldiers and prison officers remaining the primary targets. It is clear the terrorists are not concerned about the safety of anyone".
Earlier this month, dissident republicans fire bombed a shop in Belfast city centre.
Senior police officers have to date declined to speculate on the reasons for the recent rise in terrorist activity.