A special police team searched through thousands of postal items

Dissident republicans are being blamed for the devices, which were intercepted at the Mallusk postal sorting office in Newtownabbey.

It has been established that the letter bombs found early on Thursday contained petrol rather than explosives, which meant they were not identified by sniffer dogs.

One of the devices was addressed to the governor of Maghaberry Prison in County Antrim.

Four devices were sent to district policing partnership members in the north west.

A special police team searched through thousands of postal items by hand, looking for any more suspicious packages.

No warnings

Police have warned the public to be cautious when dealing with their post and asked people to contact them if anything appears suspicious.

No warnings had been received, said the PSNI.

Northern Ireland Secretary Paul Murphy condemned the attacks.

"These activities have no place in the Northern Ireland of the 21st century," he said.

The director general of the Northern Ireland Prison Service, Peter Russell, said: "The nature of such attacks is particularly indiscriminate and is to be condemned.

"Prison officers perform a very difficult role on behalf of society and they should not be intimidated in this way.

"Of course those behind this have no idea who is going to open such a package and who might be injured.

'Disturbing'

"I am thankful that due to the vigilance of postal staff this has been avoided today but we do all need to remain alert to this type of threat."

Policing Board vice-chairman Denis Bradley said it was "disturbing for the people involved".

"They are doing their best, they have taken a stance around policing - to get in there and help and make it more sensitive to the needs and desires of the community," he said.