A letter-bomb-type package was discovered at the offices of the Public Prosecution Service in Londonderry – the third of such devices found in the past week.
Two bombs were discovered at Royal Mail sorting offices last week, one addressed to PSNI Chief Constable Matt Baggott and the other to Derry area commander Jon Burrows.
The alarm was raised after the suspicious package was found yesterday morning at the Foyle Chambers offices on the Limavady Road.
The package had been delivered by an unsuspecting postal worker.
The building was evacuated so that bomb disposal experts could deal with it. The package was then removed for further examination.
A PSNI spokesman said: "Police can confirm that this package was a viable letter- bomb-type device similar in make-up to the two devices discovered on Friday.
"Police would continue to ask members of the public to remain vigilant when opening their mail and anyone who notices anything suspicious should contact the police immediately."
This incident follows a spate of recent attacks.
There were additional alerts in Derry's Waterside last Friday, as well as pipe-bombs thrown at police on Charlotte Street last Tuesday night and the following night in Newtownabbey.
SDLP Foyle MP Mark Durkan condemned those responsible for the latest incident.
He said: "Once again we have yet another example of the disruptive impact of the violent efforts of so-called dissidents.
"If this is a real letter-bomb, it is intended to damage. If it's not a real or viable device, then it is intended to disrupt and put people off their everyday business.
"Those responsible for leaving this device have shown complete and utter contempt for the people of Derry.
"They are out to destroy and don't care if they injure or kill anyone in the process.
"This callous and dangerous act flies in the face of the efforts made by so many people to improve life here.
"Thankfully no one has been injured but those graces are no thanks to those who were behind this attack."
Sinn Fein Waterside councillor Bridget Meehan also condemned the incident.
"This is a very worrying development and no doubt will have caused great concern to members of staff there and local postal staff," she said.
"This comes against a backdrop of alerts in the Top of the Hill area on Saturday and last week's incident at Charlotte Street. These incidents serve no purpose and those responsible need to come forward and explain to the people of Derry what the point of all this is."