Four people have been arrested on suspicion of involvement in a spate of attempted dissident republican letter bomb attacks, including at the army careers office in Brighton.
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 Brighton, Oxford, 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.
The package sent to the Brighton office in Queen’s Road sparked a major alert which closed a large section of the city centre with hundreds of people evacuated from shops and offices.
Counter-terrorism police and ministry of defence bomb disposal experts were called out on February 13 after the package was discovered at the army careers office.
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."