"Attacks on the mainland, particularly here in London, I think remain the holy grail for these organizations," said Martyn Frampton, professor and author of "Legion of the Rearguard: Dissident Irish Republicanism." "An attack on London is worth 10, 20 times more to them in terms of bringing attention to their cause and to the fact, as they see it, that the conflict is not over."
And ABC News Chief Investigative Correspondent Brian Ross gets a behind-the-scenes tour of security preparations with the British Transport Police.
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