Published Wednesday, 20 August 2014
Two men, aged 35 and 46, and two women, aged 21 and 44, were arrested in Derry on Wednesday morning in connection with a series of devices sent last year and earlier this year.
They are being questioned at Antrim Serious Crime Suite.
A spokesperson for the PSNI said enquiries into the devices are being led by Serious Crime Branch detectives who have been in contact with England's South East Counter Terrorism Unit.
The arrests come after packages, containing explosive powder, were sent to a number of Armed Forces careers offices in England in February - dissident republicans were blamed.
Police said they believed at the time that the 'New IRA' sent the devices to locations in Oxford, Brighton, Canterbury, Aldershot, Reading and Chatham and a shopping centre in Slough - none of which detonated.
A month later, two letter bombs were intercepted at a postal sorting offices in Lisburn and Londonderry, which were addressed to Maghaberry Prison.
Dissidents were also blamed for sending a series of letter bombs to high profile political and security figures in Northern Ireland in October last year, including Secretary of State Theresa Villiers and then PSNI Chief Constable Matt Baggott.
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