A viable letter bomb has been found during a security alert at government offices in Londonderry.
Army bomb experts were been sent to offices of the Public Prosecution Service at the Limavady Road in the Waterside area of the city on Monday morning.
Police have now confirmed the package was a viable letter bomb, similar to those found on Friday.
They have appealed for the public to remain vigilant when opening their mail.
Monday's find comes after two devices in packages addressed to senior police officers were intercepted at two Royal Mail sorting offices on Friday.
The first device was addressed to PSNI Chief Constable Matt Baggott at Knock police headquarters and was detected in the Royal Mail Mallusk sorting office.
The second was found yesterday morning in a Lisburn postal delivery office and is thought to have been addressed to Chief Inspector Jon Burrows in Londonderry.
Assistant Chief Constable Will Kerr said dissidents were believed to be behind the attacks.
Foyle SDLP MP Mark Durkan said the people responsible showed complete and utter contempt for the people of the city.
He said: "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."