Concerns have been raised over how a letter bomb addressed to the Secretary of State made it all the way to Stormont Castle.
The device sparked a major security alert and forced the First Minister from his office.
Dozens of staff at the seat of power in Northern Ireland, including Peter Robinson (right), had to be evacuated on Tuesday afternoon after a suspect parcel was found by workers in the post sorting room.
Army bomb disposal experts were sent to the scene.
They described the device as "viable" and it was taken away for further examination.
Police have said the device was "similar" to those found in the past week which have been blamed on dissident republicans.
Security sources have said the device was small and was packaged in a jiffy-type bag, similar to previous items, and that had it exploded it would have caused serious injury.
Stormont Castle is home to the Office of the First and Deputy First Minister and is also the location for meetings of the Executive.
While Secretary of State Theresa Villiers is not based in the building, her office is in the nearby Stormont House and its post is handled in Stormont Castle.
MLA Paul Girvan, who is Peter Robinson's Private Parliamentary Secretary, told the Belfast Telegraph it was a "concern" that the package managed to make it all the way through the postal system to Stormont.
He said all the mail was delivered to a central reception area in the building and that it would have been close to the First Minister who was working in his office ahead of the alarm being raised on Tuesday morning.
"It's in the same building. There would have been a lot of people in that area," said the DUP representative.
"There should be special attention given to parcels addressed to high-ranking officials like the Secretary of State, especially given the latest spate of these incidents and everyone has to be more vigilant.
"I would expect there to be a serious study going on, by the police, security at Stormont and those involved with the post as to how this managed to happen."
The Department of Finance and Personnel, which runs the Stormont estate, said it was investigating the incident to determine if any further action was required.
Royal Mail, which handles some of the post at Stormont and discovered letter bombs in its sorting offices last week, said it had "strict procedures and protocols" in place to deal with suspect packages.
A spokeswoman said: "We have strict procedures and protocols in place to deal with suspect devices and trained personnel who manage these situations on the ground.
"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."
Ms Villiers was in London at the time of the alert, meeting with US diplomat Richard Haass, and she condemned the attacks.
First Minister Peter Robinson said that had the bomb gone off the victims would have been postal workers, office staff and civil servants. "Do they really think the Secretary of State opens her post," he asked.
"The attack might get more publicity because of who it was addressed to but the impact would have been on a public servant."
Those behind the terror bid "will not further any aim or objective by their vile and callous deeds", he said.
Deputy First Minister Martin McGuinness described the incident as "the offerings of bitter and twisted little minds" and their actions "will further nothing".
Justice Minister David Ford, who happened to be visiting a Royal Mail sorting centre yesterday to praise postal staff for their diligence over recent days in discovering the letter bombs, added his condemnation.
The parcel sent to Theresa Villiers is the fourth letter bomb device which has been intercepted in the past week after similar devices were sent to the Chief Constable Matt Baggott, John Burrows, Londonderry's highest-ranking police officer, and the offices of the Public Prosecution Service in Derry.
Those were blamed on dissident republicans and police said yesterday's device was "similar in make-up to the devices found in recent days".
They have appealed for information.