The letter bomb discovered in Derry’s postal sorting office yesterday is believed to have been to a person in Magheraberry Prison.
A second device found in a Royal Mail office in Lisburn this morning was also addressed to the prison.
The Derry device, discovered in the postal sorting office in Great James Street, was in A4 Jiffy-type bag with an address in a grey stencil.
The attacks have been widely condemned.
In a joint statement, First Minister Peter Robinson and Deputy First Minister Martin McGuinness said those who sent the packages had no respect for postal workers, the wider community or the future.
Police have warned the public to be vigilant and to contact them if they receive a similar package.
