An extremist Irish nationalist group has claimed responsibility for sending explosives to the U.K.

An extremist Irish nationalist group calling itself the Irish Republican Army has claimed responsibility for sending five explosive devices to different places in the United Kingdom last Tuesday.

Parcels capable of "igniting small fires" were found at London's Heathrow and City airports, and another at Waterloo train station, the country's busiest train hub, reports The Guardian. A fourth device was also sent to the University of Glasgow, where police orchestrated a controlled explosion. The fifth device, if it exists, has not been discovered. The three London devices did not ignite and no one was injured in any of the cases.

Metropolitan police began investigating the origins of the devices last week. The packages were labeled with Irish stamps that read "Love Eire N." (Eire is the Irish word for Ireland.)

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On Monday, the Irish News received a tip using a recognized IRA code word, in which the dissident group claimed responsibility for the bombs. Police in the U.K. were already considering the possibility of IRA involvement — specifically a group known as the New IRA, a splinter group that has remained intermittently active since rejecting the terms of the Good Friday Agreement, a ceasefire and peace treaty signed by Ireland, the U.K., and numerous factions within both countries (including the IRA), which was signed in 1998 following three decades of conflict.

RTE reports that Gardaí, the Irish police force, believes the New IRA is increasing its activity in Northern Ireland (which remains part of the U.K.), citing the recent the incidents in the U.K. and a car bomb that exploded in Derry, Northern Ireland.

There is no specific indication in either scenario as to why the New IRA may be resurfacing, though it is worth noting that there are fears that Brexit could reawaken the dormant conflict in Northern Ireland.