BELFAST, Northern Ireland (AP) — Northern Ireland police say they have arrested four people suspected of organizing an Irish Republican Army letter-bomb campaign.

Hours after Wednesday’s arrests in Londonderry, masked men loaded a suspected bomb on to a truck and ordered the driver to park it outside the city’s police headquarters. Police evacuated a nearby school and businesses but could not confirm whether the threat was genuine.

Those arrested — men aged 46 and 35, and women aged 44 and 21 — are suspected of mailing at least 10 bombs over the past year. All were detected and dismantled safely.

Targets in Northern Ireland and England included police commanders, a prison governor, a state prosecutor’s office, British Army recruitment centers, and Northern Ireland’s secretary of state.

Several small IRA factions remain active in Northern Ireland.

Copyright 2014 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.