Ms Villiers was targeted by a viable device intercepted at Stormont Castle on Tuesday - the fourth sent in the last week.
Police believe dissident republicans are behind the attacks on the Conservative MP and senior police and the offices of the Public Prosecution Service in Londonderry.
The Secretary of State was in Belfast to meet with city centre management over the threat of more loyalist flag protests in the lead up to Christmas.
"A number of people could have been injured or killed as a result and I've concerned about that," she said.
"That's why I went to a postal sorting office this morning, that's why I also had a chance to meet the staff at Stormont Castle, who found the envelope that was addressed to me.
"I wanted to say thank you to them and give them reassurances that all focus is on trying to track down the people responsible for these attempts."
She added that the incidents are "economically damaging" and that those behind them would "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," she said.
Ms Villiers said that police on both sides of the border are continuing to crack down on terrorist activities in the face of a severe threat.
She emphasised that the British Government would do all it could to support the efforts of the PSNI.
A suspect parcel which sparked a security alert on Wednesday was found to be a false alarm.
A postal sorting station at Stormont House was evacuated after the package raised concerns among staff but it was found to be a genuine item for a member of staff.
A PSNI spokeswoman appealed for the public to remain vigilant and to report anything suspicious to police immediately.
Earlier on Wednesday, police were called to the law courts in Belfast after a package raised concerns but it was also found to be a false alarm.