Five letter-bombs addressed to members of District Policing Partnerships and the governor of Maghaberry jail have been intercepted.
The devices, described as "viable", were discovered at Royal Mail's Mallusk sorting office in Co Antrim early yesterday. They contained petrol rather than an explosive and had not been detected by sniffer-dogs. The discoveries prevented the transfer of mail to 26 regional sorting offices across Northern Ireland. Household and business deliveries were badly disrupted.
Police officers are understood to have searched thousands of postal items by hand looking for additional devices.
The PSNI claimed no warning had been received and called on the public to be vigilant.