Police in the North have urged the public to be careful when opening mail today.
The warning comes after parcel bombs addressed to the a prison governor and two District Policing Board members were discovered in Northern Ireland 's main postal sorting office overnight.
Postal staff discovered the bomb and two other devices at the Mallusk sorting office in Co Antrim.
One was addressed to a governor at Maghaberry Prison, near Lisburn, Co Antrim, jail authorities confirmed. The other two were addressed to two members of the District Policing Partnership Board.
Mr Peter Russell, director general of the Northern Ireland Prison Service, said: "Those behind this have no idea who is going to open such a package and be injured.
"The nature of such attacks is particularly indiscriminate and is to be condemned.
"I am thankful that, due to the vigilance of postal staff, this has been avoided today, but they need to remain alert to this type of threat," he said.
All three devices were defused by army bomb experts.
The packages were discovered at various times during the night as postal workers at sorted through the mail.
After the first two devices were found and made safe by the British army specialists, staff returned to work but when the third suspect package was discovered and the centre was evacuated again and closed down.
A full-scale search of the premises is still being carried out. A police spokesman described the first two devices as "crude but viable".
The PSNI have also put out a warning for everyone across Northern Ireland to be on their guard when opening post today and said anything suspicious should immediately be reported.
The Royal Mail warned there would be widespread disruption to deliveries. No warnings had been received.
Elsewhere, bomb-disposal experts were called to a post office in the Diamond at Portstewart, Co Derry, after a suspect device was found.