DUBLIN (Reuters) - Irish police said on Monday they had released a man arrested over an alleged plot to murder up to a million people by poisoning England‘s water supply unless British troops were pulled out of Northern Ireland. Police said the man, in his 50s, had been released without charge. A file had been sent to the director of public prosecutions, a spokeswoman said. The man, originally from Scotland, was arrested on Saturday in Dublin after a month-long investigation by British and Irish police into an alleged plot to dump weed killer into drinking water supplies. The arrest was made as Northern Ireland began a crucial week of trying to salvage last year‘s Good Friday peace agreement that seeks to end decades of sectarian violence between the Protestant majority and the Roman Catholic minority. Britain‘s government said it kept silent about the threat until the man‘s arrest for fear of causing panic or encouraging imitators. The Cabinet office said it did not believe the plan could have been fully carried out or would have put people in serious danger, but said the threat "displayed enough technical detail to give cause for concern". In letters sent to officials including Prime Minister Tony Blair the man said he was from a previously unknown Irish group called "The Republican Revenge Group" and demanded that Britain pull its 17,000 troops out of Northern Ireland by June 18. The letters threatened to kill "up to one million" people.