Four people in Londonderry were arrested on Wednesday (August 20) in relation to a series of viable letter bombs sent to addresses in Northern Ireland and England last year.
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.
The PSNI didn’t release details of the specific incidents nor the locations where the alleged offences being investigated are believed to have occurred.
Last year there was a spate of letter bomb incidents across Northern Ireland and England, including in Londonderry.
For example, last October the bomb squad was tasked to a security alert at the PPS office in Londonderry after the PSNI confirmed that a package delivered to the Limavady Road was a viable letter bomb type device.
Letter bombs targeting Chief Constable Matt Baggott, prominent Londonderry officer Jon Burrows and Secretary of State Theresa Villiers were also intercepted in October 2013.