Four people have been arrested on suspicion of involvement in a spate of attempted dissident letter bomb attacks last year and earlier this year in Northern Ireland and England.
The two men, 35 and 46, and two women, 21 and 44, were detained in Derry and have been taken to the PSNI Serious Crime Suite in Antrim.
One of the devices was sent to Stormont and addressed to Northern Ireland Secretary Theresa Villiers.
Another bomb was delivered to the offices of the Public Prosecution Service in Derry.
Two explosive packages - one addressed to then PSNI chief constable Matt Baggott and the other to one of his senior commanders - were intercepted at Royal Mail offices in Belfast and Lisburn.
A number of devices were also sent to British army recruitment offices in Oxford, Brighton, Canterbury, Aldershot, Reading and Chatham and the Queensmere shopping centre in Slough in February.
None of the devices detonated.
The PSNI said its detectives had been liaising with officers from the South East Counter Terrorism Unit in England.