Police arrested a man Saturday linked to a plot to kill up to a million people by poisoning England's water supply unless British troops were pulled out of Northern Ireland, authorities said.
Irish police said the man, in his 50s, was arrested at a Dublin address after a monthlong investigation by British and Irish police into the plot, which involved dumping weed killer into fire hydrants.
The arrest was made as Northern Ireland headed for a crucial week in trying to salvage last year's Good Friday peace agreement that seeks to end decades of sectarian violence between the Protestant majority and Roman Catholic minority.
Britain's government said it kept the threat under wraps until the man's arrest for fear of causing public panic or encouraging imitators.
While stressing that it did not believe the plan would have been fully possible or would have put people in serious danger, Britain's Cabinet office said the threat "displayed enough technical detail to give cause for concern."
"[Officials] concluded that, even if a threat was carried out, the risk to people, including the very old and the very young, was negligible," a statement from the Cabinet office said.
"The people most at risk were kidney dialysis patients who need a great amount of water."
The man claimed to be from a previously unknown Irish republican group called the Republican Revenge Group and demanded that Britain pull its troops out of Northern Ireland by mid-June.
Letters were sent to the British ambassador to Ireland, the Papal Nuncio in Dublin and Britain's water authority. One of the three letters also said a letter, which was never traced, was sent to Irish Prime Minister Bertie Ahern.