4 suspects arrested over IRA letter bombs; Northern Ireland truck-bomb alert follows arrests

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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.