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LONDON — Britain boosted security at ports and other border entry points in response to the massacre at French satirical magazine Charlie Hebdo. The "U.K. threat level ... remains at severe," Home Secretary Theresa May told journalists after an emergency Cabinet-level security meeting. "That means a terror attack is highly likely and public should remain vigilant."

Wednesday's attack was a reminder of the danger terrorism poses to Britain, according to the head of the country's domestic intelligence service. "It is too early for us to come to judgments about the precise details or origin of the attack but it is a terrible reminder of the intentions of those who wish us harm," MI5 Director General Andrew Parker said in a separate statement. "As you would expect, we are offering our French colleagues our full support as they respond."

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— Michele Neubert and F. Brinley Bruton