BREAKING: From the Guardian:
A major terrorist plot to allegedly blow up aircraft in mid-flight was thwarted in a joint intelligence-led operation...UPDATE: Fox News reporting 20 planes targeted as part of a 9-month plot; 20 arrested. [MSNBC reporting "at least 13" arrested.]
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It is believed that the aim was to detonate explosive devices smuggled on board the aircraft in hand luggage, and that the attacks would have been particularly targeted at flights from the UK to the USA.
UPDATE II: British terror threat level at "Critical" (Terror Attack Imminent); U.S. raises threat level to RED for incoming British flights, ORANGE for all other flights.