A letter bomb has been sent to the Public Prosecution Service office in Londonderry.
The building on the city's Limavady Road was evacuated at about 12:30 BST on Monday.
Army bomb disposal experts examined the device and declared it viable. It has now been taken away from the area.
Police said the bomb was similar in make-up to two devices found in postal sorting offices in County Antrim on Friday.
One was addressed to the chief constable and the other to another senior police officer.
Dissident republicans were blamed for sending the bombs.
A police spokesperson said: "We urge the public to be vigilant when handling post."
Justice Minister David Ford said: "Yet again we see members of our community put at risk as a result of the actions of faceless individuals who represent no one and have absolutely nothing positive to offer.
"They, and anyone who supports them in any way, are to be condemned in the strongest possible terms."
DUP Deputy Mayor Gary Middleton said: "I totally condemn those who have left the device.
"The cowards who sent this have put the postman and staff lives at risk."
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