Scotland Yard said on Tuesday a media outlet in Northern Ireland received a statement purportedly from the group using a “recognised codeword”.
It said it was behind devices sent to Heathrow and London City airports, Waterloo station and the University of Glasgow, while claiming a fifth bomb had not been discovered.
What is the New IRA?
The New IRA is a republican paramilitary group based in west Belfast but also active in the rest of the UK and the Republic of Ireland, which aims to bring about a united Ireland.
The group can trace its lineage back to the Real IRA, which was formed following a split in the Provisional IRA by dissident members who rejected the 1997 ceasefire.
It was reported in 2012 the Real IRA had merged with several smaller Republican militant organisations and – although the group calls itself as the Irish Republican Army – became widely referred to as the New IRA.
What are their objectives?
The New IRA’s primary goal is to bring an end to British rule in Northern Ireland through the use of paramilitary violence.
The group rejects the Good Friday Agreement, which effectively brought about the end of The Troubles by setting out both Northern Ireland’s status within the UK and its relationship with the Republic of Ireland.
Have they carried out attacks in the past?
The New IRA has been linked to a number of incidents in recent years. In 2014 the group sent seven letter bombs to British Army recruitment offices in southeast England – the first Republican attack on the mainland UK since 2001.
In May 2016, the group were blamed after three men were shot in paramilitary-style attacks in Republican areas of Belfast in less than 24 hours, leaving two injured and one dead.
The New IRA is considered the “main line of enquiry” in a car bomb attack on a courthouse in Derry, which took place in January this year.
How big are they?
Being a designated terrorist organisation, there are no concrete figures for how many members the New IRA currently has.
However, MI5 and the Police Service of Northern Ireland (PSNI) believe the group remains small, focussed around a core of no more than a few hundred people.
How much of a threat are they?
Although the Real IRA was known to target the UK mainland with car bombs and incendiary devices, the group remains most active in Northern Ireland.
MI5 says although the risk of attacks by dissident republicans persists, the vast majority of Northern Irish people can go about their daily lives untroubled by terrorism.
The Security Service currently deems the threat of Northern Ireland-related terror to be “severe” in Northern Ireland.
The threat level in Great Britain had been lowered from “substantial” to “moderate” a week before the first letter bombs were found at London transport hubs.