Four people have been arrested as part of an investigation into attempted letter bomb attacks in Northern Ireland and England earlier this year.
Two men and two women were detained in Derry this morning.
A number of the devices were mailed to army recruitment offices.
Devices described as crude but potentially explosive were mailed to seven army offices in southeast England in February.
Four devices were discovered at army careers offices in Oxford, Brighton, Canterbury and the Queensmere shopping centre in Slough.
Others were sent to Aldershot, Reading and Chatham.
Last October, another series of letter bombs were sent to high profile political and security figures, including one addressed to Northern Ireland Secretary Theresa Villiers at Stormont Castle in Belfast.
None of the devices exploded.
