Four people have been arrested on suspicion of involvement in a spate of attempted dissident republican letter bomb attacks in Northern Ireland and England, including on a number of Armed Forces recruitment offices.
The two men, 35 and 46, and two women, 21 and 44, were detained in Londonderry and have been taken to the Police Service of Northern Ireland's (PSNI) Serious Crime Suite in Antrim.
They are being questioned by detectives investigating the sending of explosive devices to Armed Forces careers offices in Oxford, Brighton, Canterbury, Aldershot, Reading and Chatham and the Queensmere shopping centre in Slough in February.
In October last year dissident republicans opposed to the peace process were also blamed for sending a series of letter bombs to high profile political and security figures in Northern Ireland.
None of the devices detonated.
In the Northern Ireland attacks, one of the devices was sent to the seat of power sharing executive at Stormont Castle in Belfast addressed to Northern Ireland Secretary Theresa Villiers.
Another bomb was delivered to the offices of the Public Prosecution Service in Londonderry while 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.
The sending of bombs in the post marked the re-emergence of a terror tactic that was used by paramilitaries during the Troubles.
Since 2009, republican extremists intent on achieving a united Ireland by violent means have murdered two soldiers, two policemen and one prison officer in Northern Ireland, as well as carrying out numerous other attacks on security force targets.
The PSNI said its detectives had been liaising with officers from the South East Counter Terrorism Unit in England.
A PSNI spokesman said: "Police investigating a series of devices sent to addresses in Northern Ireland and England last year and earlier this year have arrested four people in Londonderry this morning.
"Two men aged 35 and 46, and two women aged 21 and 44, were arrested in the city and taken to the Serious Crime Suite at Antrim police station for interview.
"The investigation is being led by detectives from PSNI Serious Crime Branch who have been working in close liaison with officers from the South East Counter Terrorism Unit in England."
In March this year two more letter bombs were intercepted at postal sorting offices in Lisburn and Londonderry.
Both were addressed to Maghaberry high security prison in Co Antrim.