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Police officers with automatic weapons and bomb-sniffing dogs patrolled at U.K. airports and transportation hubs Friday after the government raised the country's terrorism threat level from "substantial" to "severe." Security officials said the alert, announced for airports, subway and train stations, was designed as a precaution, and was not based on any specific intelligence, though BBC News reported it had seen a letter saying al Qaeda might be planning an attack. The general terror alert for the whole country has been at "severe" for a whole year, according to Scotland Yard. In December, British police conducted coordinated raids that nabbed 12 young men suspected of terrorism.